How to make iOS slow

I'm a mobile QA. Now we have an issue about a race condition between network response and UI rendering. We guess if the UI rendering is slower than the network response, then it will crash.

We already tried to speed up the network response, by using Charles' map local feature. But the duration is still about 20ms. This is the best way we can think to speed up the network.

So I'm asking if there is any way to slow down the UI rendering on iOS, real device or simulator. Is there a way to set the CPU usage or memory for iOS? Or if there is a way to keep the iOS system in high CPU / memory usage?