ios - Performance issues when using UIModalPresentationOverFullScreen vs UIModalPresentationFullScreen -

i have complex, static, viewcontroller a presents viewcontroller b on it. b covers a , opaque.

i noticed if present b using uimodalpresentationoverfullscreen fps drops around 30, if use uimodalpresentationfullscreen fps closer desired 60. know since b being presented opaquely on a doesn't make sense use former style, but i'm curious know why rendering engine still struggling since far knows, doesn't need render/update a.