In my case, GIT integration appears to be causing the editor to be frustratingly slow with 13.
While typing, even comments, with GIT integration turned on, after about 30 characters, the UI freezes for a second or so. Its usually not long, but very annoying.
I am using GIT 1.7.8.0. Running on Windows 7 64 with a solid state drive and 12 gigs of ram and an intel I7 with 8 CPUs. I tried various things, like updating the idea64.exe.vmoptions to use more memory, like -Xmx2400m and -XX:MaxPermSize=2400m, -XX:ParallelGCThreads=6, but it didn't fix the problem.
The git repository is 1.3 gigs with 65,000 files.
I created a new "grails" project in a new git repository, and there is no issue. I created a new grails project in the existing large git repository, and intellij is slow. I turned off git integration by opening the project settings dialog and deleting the git root, and the problem goes away.
I tried disabling all of the GIT background operations through the 13 UI, but it didn't make a difference. I also tried both GIT built-in and native modes, and it made no difference.
In my case the workaround seems to be to disable GIT integration until I need it, and to then just re-add the git root. If anyone else can verify the same problem, then we might report it as an issue.