Strange performance issues with new versions of Monogame/Xamarin

We develop small 2d games for iOS. We use intensively SpriteBatch for sprite rendering.

Several months ago things were fine: fps nearly 60 in debug/release modes on different devices (iPhone 4s, 5, 5s, 6). But after updating of Monogame (3.2 -> 3.3 -> 3.4) and Xamarin (latest versions) we faced with inexplicable problems.

First of all fps in debug mode fell on the order of magnitude (e.g., ~55 fps -> ~15 fps on iPhone 4s). Also appeared random performance drops in release mode.

Obviously question is sparse. Simply it would be interesting to hear what changes in Monogame codebase (potentially in Xamarin) could lead to this result.

Profiling gave nothing. Thanks in advance.