Problems after upgrading to latest Unity version

My copy of Unity decided to auto-update itself from 5.1 to the latest version, after which my framerate was almost cut in half, some objects using the Standard shader now flash constantly, and all the meshes I had generated via an editor script were removed from their GOs and can’t be replaced via the editor (apparently because I had saved them as .mesh files rather than .asset files and assigned the mesh via Editor script, but this used to work). And this comes at a time when I’m desperately in a hurry to get my current project done.
Anyone know why the framerate would drop so much? That may be the largest headache overall, since I’m going to need to record gameplay footage soon and that cuts the framerate even more, obviously. The strange thing is that the new framerate is only halved while in game mode but not when looking at the game window in editor mode; and in game mode it remains almost constant regardless of how much stuff is in view, whereas it used to fluctuate based on context. Task Manager shows that there is very little drain on the system overall - much less than usual - and Unity is also using less than usual.

It seems that this is a common problem when upgrading to 5.3: performance - Android Low fps after Unity 5.3 update - Stack Overflow
Has anyone found a workaround? A Google search didn’t help.