GPU Graphics.blit huge time on android

Hello to everyone!
I'm a beginner in Unity, and I'm making a stealth game for android. However the game has horrible performance. Using the Profiler in Unity Editor (connected to my smatphone) I found some things. First of all, my game has two views: top-down and third pearson.
In top-down framerate is about 15 fps, while in third person it's about 30 (the game runs pretty smooth).
In top-down GFX.WaitForPresent takes about 80% of the time. Searching on Internet I enabled GPU profiling. I found that Camera.Render > Drawing > CommandBuffer.Before... > Ambient Occlusion > Graphics.Blit takes about 60% of time (112 ms!).
Searching in web I found that Graphics.Blit is releated to Post Processing but I may be wrong. In any case, I didn't set up any Post processing effect!
In GPU profiler graph most of GPU work is colored as "Other".
Any suggerstion? Thank you very much!