The project had shown serious fps degradation when working under iOS11 (it was working as expected under iOS10).
Devices, where the problem is most apparent: iPad mini 2, iPad mini 3, iPad Air 1.

CPU spikes in Unity Profiler were noticed (they were not present under iOS10).
Yet, XCode doesn’t show any significant changes in FPS, but they are nonetheless present, visible in the form of constant frame skips. XCode profiler doesn’t show any changes in CPU / GPU general load either.

Steps:
1. Build and Run user's project on iOS11 device (use development build)
2. Attach Unity Profiler
3. In device: toggle the post-effects stack by pressing “PostFX disabled”
4. Compare results with the same device running iOS10

Result: iOS11 payer skips frames (although, fps count stays the same), spikes in Unity Profiler.

Tested with:
iPad Air iOS9.1
iPad Air iOS11.0