Latest unity version caused super-massive-giant lag

I had Unity 3.5.4.something for a while now, and I just got the latest version today in addition to the 30-day trial.

My game just went from more than 100 fps to 1.8 fps. I disabled my terrain collider and it was at 80-100 fps, so apparently this stupid update can’t handle having many instanced-trees in the game.

Thank god I opened this with a backup, and why is this happening? I’m really annoyed because now I just used up my 30-day license on making my game completely unplayable. What am I suppose to do now? Downgrade?

Maybe, and only maybe, some scripts have old references and the console pops out them causing it the lag.