I believe the point is that the "main thread" for your iOS application, is not the main thread of the OS. They're totally decoupled.

Err, same with Android? And every OS ever. That's just standard process isolation. Or am I misunderstanding something?

There’s some deep dive articles on the way the input loop works. But OP is correct, and the reason iOS feels smoother. Android has a lot more UI lag.

I don't think Apple has any special tricks for input loop.

Some Android phones really have input lag, but it is not caused by CPU load. For example, on my phone, there is approximately 100-150 ms lag between tapping the screen and registering the touch. The lag is not caused by CPU load, but by slow touch sensor.

I don't think Apple has any smart code optimization tricks. Either they have a faster touch sensor or just some people believe that if something is made by Apple then it is of a better quality.

Here is a comparison of input lag in games in iOS and Android [1] and it shows similar results: 80 ms between a tap and reaction on screen.

[1] https://blog.gamebench.net/touch-latency-benchmarks-iphone-x...

They do! See https://devstreaming-cdn.apple.com/videos/wwdc/2015/233l9q8h... and the corresponding WWDC session on them minimizing the input-to-display time.

This is off-topic, but I love that the title of the slides is "233_Advanced Touch Input on_iOS_04_Final_Final_VERY_Final_D_DF.key".

<laughs in Windows 3.1>

UIView animations do not run on the main thread, and will not be blocked if the main thread is blocked. This does help a bit with keeping the OS feeling smooth, but it is far from the only reason.