All changes you can find on page: http://jetbrains.net/confluence/display/SCA/Version+History
Just two hints:
1) If you have only small files, you can use Implicit-Aware completion feature, but if not, while we fix perfomance for this feature, I can suggest to turn off this:
File -> Settings -> Code Style -> Scala -> Other Settings -> Check implicit conversions...
2) If you still have perfomance problems, try to use Diana, not Maia. Diana twice faster for now.
And last, if you still have perfomance froblems, you need to wait next releases, I'm going to improve plugin speed.