You can change it to the old theme on your settings page (to steal Antivirus's post):
Profile -> Modify Profile (top left above Summary) -> Look and Layout -> Click "Change" in the Current Theme row -> Click Use This Theme for the theme called Core Theme -> Click Change Profile (bottom right) -> Done!\
I hate change, but I'm gonna try and get used to the new theme. We'll see if I last.
This theme definitely feels wayyy faster, or maybe it's just SMF 2.0 that's faster.