It's been like this forever really. Because I worked both on Ubuntu and MacOSX interchangeably I didn't care as much. Now I think maybe it's time to finally find out what is wrong with it.
THE PROBLEM: when I switch tabs or navigate the lines with arrows or J,K keys and when I do it repeatedly (as if when trying to find the right tab holding Ctrl and hitting Tab) it seems to be delaying the action. When I stop hitting the keys gvim may still be finishing the operations (for instance, I stop hitting Ctrl+Tab, gvim may switch 5 more tabs - the number depends on how long I've been hitting Ctrl+Tab - before it stops).
MacVim with similar settings in .vimrc works just fine. The delays are present both when Compiz is turned off and turned on. Any ideas?