What Makes an App Run Fast on Different Phones?

Have you ever noticed how some apps open in less than a second while others keep you staring at a loading screen? The speed of your app can be the difference between someone using it every day or deleting it after their first frustrating experience, and after building apps for over ten years, I've learned that performance isn't something you can fix at the end of development... it needs to be part of your thinking from day one. The phones in people's pockets range from brand new flagships costing a grand to three-year-old budget models that cost around eighty quid, which means your app needs to work well for everyone, not just the people with the latest technology.

App performance is the silent killer of user engagement, most people won't tell you why they stopped using your app, they'll just quietly uninstall it

What we're going to look at here is the practical side of making apps run quickly across different devices, from how phones actually process your code to the mistakes that trip up even experienced developers, and I'll share what we've learned from building apps that need to work on everything from the newest iPhone down to Android devices that are several years old.

Why Some Apps Feel Lightning Quick While Others Lag

The difference between a fast app and a slow one usually comes down to three main things: how much work the app tries to do when it first opens, how efficiently the code is written, and whether the developers tested on older phones or just the expensive devices they have on their desks. When someone taps your app icon, they expect to see something useful within two seconds at most... anything longer than that and you've already created a negative feeling that's hard to shake off.

I've worked on apps where the team had built everything on new iPhones and wondered why they were getting complaints. The app loaded in one second on their devices.

On a two-year-old Android phone? Seven seconds.

The problem was that they were loading every single feature and piece of data right at the start, even things the person might never use, which brings us to the concept of lazy loading... only bringing in what you need when you actually need it, not all at once. This type of performance optimization and scalability testing helps identify these bottlenecks before they impact users.

• Apps that load their entire database into memory at startup instead of fetching data as needed
• Using high resolution images without compression or different sizes for different screens
• Running complex calculations on the main thread which freezes the interface
• Making dozens of separate network requests instead of batching them together
• Not caching anything which means downloading the same data repeatedly

How Different Phones Process Your App

The technical differences between a budget Android phone and a flagship iPhone are massive when you look at what's happening under the bonnet. A top-end phone might have eight processor cores running at speeds over 3GHz with 8GB or more of memory, while a cheaper device could have four slower cores and just 2GB of memory to work with, which means the same code runs at completely different speeds depending on the hardware.

The way Android and iOS handle apps is different too, which catches out developers who only think about one platform. iOS manages memory quite aggressively and will shut down your app in the background if it needs space for other things, so you need to save the person's state properly or they'll lose their place. Android gives you more flexibility but that also means more variation between devices because manufacturers like Samsung or Xiaomi add their own modifications to the operating system. Understanding these nuances is important whether you're building a native app or considering progressive web app alternatives.

Always test your app on a device with 2GB of RAM or less, even if your target audience typically has better phones, because this quickly exposes memory problems that might not show up on more capable hardware

Memory Management and What Happens Behind the Scenes

When your app is running, it's constantly using memory to store information... the images on screen, the data it's downloaded, the person's login details, and dozens of other things. The phone only has so much memory available and it needs to share that between all the apps running at the same time, so if your app is greedy with memory, the operating system will either slow it down or shut it down completely.

The mistakes I see most often involve keeping things in memory that aren't needed anymore. We worked with an e-commerce client whose app would crash on older phones after people browsed about twenty products, the problem was that every product image was being loaded at full resolution and kept in memory even after the person scrolled past it, which meant memory usage just kept climbing until the operating system killed the app.

The fix was straightforward once we identified it... clear images from memory when they're no longer visible, load smaller versions for thumbnail views, and only download full resolution when someone actually looks at the product details. Memory usage dropped from over 400MB to around 80MB, the app stopped crashing. This kind of optimization is crucial for apps that handle rich media content, similar to the considerations needed when handling different audio qualities in streaming applications.

Making Your Code Run Faster on Older Devices

Writing code that performs well on older phones means thinking differently about what work you're asking the device to do. Older processors are slower so any unnecessary calculations or complex operations become much more noticeable, and you need to be careful about running things on the main thread because that's what controls whether your interface feels smooth or janky.

The main thread is like the conductor of an orchestra (without that I'd need another way to explain it)... sorry, what I mean is the main thread handles all the visual updates and touch responses, so if you block it with heavy work, the whole app freezes and feels unresponsive. Anything that takes more than a few milliseconds needs to happen on a background thread instead, things like processing images, sorting large lists, or parsing JSON data from your server.

Performance on older devices isn't about cutting features, it's about being smarter with how and when you execute your code

We built a healthcare app that needed to work on older devices in clinics that hadn't updated their hardware in years. The app processed patient forms with lots of calculations, which took several seconds on old iPads and made the interface completely unresponsive. Moving those calculations to background threads meant the interface stayed smooth and people could keep using other parts of the app while the processing happened quietly in the background, total processing time didn't change but the perceived performance improved dramatically. When building apps for international audiences, this type of performance consideration becomes even more critical as different countries may have varying device capabilities.

Network Speed and Data Loading Strategies

The speed of someone's internet connection varies wildly... they might be on fast WiFi at home, spotty 4G on the train, or barely functioning 3G in a rural area. Your app needs to handle all of these situations without making people wait or showing endless loading spinners, which means being thoughtful about what data you request and when you request it.

The biggest wins come from three strategies: caching data locally so you don't need to download it every time, loading the most needed information first and less important things afterwards, and making your API responses as small as possible by only sending the data you actually need. A fintech app we worked on was sending complete transaction histories every time someone opened it, including fields that were never displayed in the mobile interface.

Reducing the API response to just the fields needed for the mobile view and caching the transaction history locally cut the data transfer from around 850KB down to about 120KB per load. For someone on a slow connection, that changed the load time from eight or nine seconds down to under two seconds. These optimizations become even more important when you're integrating payment systems and checkout flows where speed directly impacts conversion rates.

• Implement aggressive caching for data that doesn't change often like product catalogues or user profiles
• Use pagination to load large lists in chunks rather than everything at once
• Compress images on your server before sending them to mobile devices
• Show cached content immediately while fresh data loads in the background
• Add retry logic that backs off gracefully when network requests fail

Testing Across Different Phone Models

You can't know how your app performs on different devices unless you actually test it on those devices, yet I've seen teams launch apps after only testing on the two or three phones sitting in their office. The performance differences between devices are too significant to guess at or assume everything will be fine, you need real data from real hardware.

The minimum testing setup should include at least one older iPhone (maybe an iPhone 8 or similar), one budget Android device with 2GB of RAM, one mid-range Android from a major manufacturer like Samsung, and whatever flagship devices are popular in your market. This gives you coverage of the main performance profiles you'll encounter in the real world. When conducting this testing, it's also valuable to analyze how competitor apps perform on the same devices to understand performance benchmarks in your market.

Use remote testing services like Firebase Test Lab or BrowserStack when you can't afford to buy every device yourself, these services let you run your app on hundreds of real devices in the cloud and see exactly how it performs

Common Performance Mistakes That Slow Apps Down

After working on dozens of apps, you start seeing the same performance mistakes come up again and again. These aren't usually caused by bad developers, they happen because performance issues aren't obvious until you look for them, and they often creep in gradually as features get added to an app over time.

One of the most frequent problems is loading too much data at startup. Teams add a new feature that needs some data, then another feature that needs different data, and before long the app is downloading megabytes of information before it even shows anything to the person using it. Each feature on its own seemed reasonable but together they create a terrible first experience. This is particularly challenging for super apps that combine multiple services where the temptation is to load everything upfront.

Another common issue is not optimising images properly... someone exports an image at full resolution from Photoshop, drops it into the app, and doesn't think about the fact that it's three thousand pixels wide but only displays at three hundred pixels on screen. You're making people download huge files that then need to be shrunk down, wasting bandwidth and processing power. For luxury brands especially, where image quality is crucial, finding the right balance requires careful consideration of development costs versus performance optimization.

1. Loading entire databases into memory instead of querying what you need
2. Making network requests in loops instead of batching them together
3. Not implementing proper image caching which means redownloading repeatedly
4. Running animations at 60 frames per second when 30 would look fine
5. Keeping video or audio loaded in memory when not actively playing
6. Using expensive operations inside frequently called functions

How to Monitor and Improve Speed Over Time

Performance isn't something you fix once and forget about, it degrades over time as you add features, update libraries, and change code. You need ongoing monitoring to catch problems before your users do, which means instrumenting your app with performance tracking and actually looking at the data regularly.

We use tools like Firebase Performance Monitoring or New Relic Mobile to track things like app startup time, screen rendering speed, and network request duration across all the devices using our apps. This data gets reviewed every Tuesday morning in our team meeting because performance needs to be an ongoing conversation, not something you only think about when complaints come in. This ongoing monitoring approach aligns with broader cloud performance and cost management strategies that help maintain optimal app performance.

The apps that maintain great performance over years are the ones where the team treats speed as a feature that deserves the same attention as any other part of the user experience

Set performance budgets for different parts of your app... your home screen should load in under two seconds, your search results should appear in under one second, your checkout flow should never take more than five seconds to process. When monitoring shows you've exceeded those budgets, that becomes a priority to investigate and fix before shipping the next update. For social media apps, where user engagement is critical, these performance budgets become even more important when considering the investment required for optimal user experience.

Conclusion

Making an app run quickly across different phones isn't about one magic technique, it's about dozens of small decisions throughout the development process that add up to an experience that feels fast and responsive regardless of what device someone is using. The phones in people's hands have wildly different capabilities and your app needs to respect that by being efficient with processor time, careful with memory usage, and thoughtful about network requests.

The performance work we do for clients often isn't visible in the same way that design changes are, nobody notices when an app is fast, they just enjoy using it and come back more often. They definitely notice when it's slow though, and they don't usually give you a second chance to fix it... they just move on to a competitor's app instead.

What matters is building performance thinking into your process from the beginning, testing on real devices that represent your actual users, and monitoring what's happening in production so you can catch problems early. The technical side isn't as complicated as it might seem, it just requires paying attention to how your code actually runs on the hardware that matters.

If you're working on an app and need help making sure it performs well across different devices, we'd be happy to have a chat about your project and share what we've learned from building fast, reliable apps for the past decade.

Frequently Asked Questions