I havent seen this issue in some time, but after doing a clean 11.2 install, on one of my machines, the context menus and dropdowns -particularily in firefox- were very slow in displaying. I thought maybe it was a firefox issue so I reverted from 3.6b3 to 3.5, but the problem still remained.

I found this nvidia tweak fixed the issue -

Code:
nvidia-settings -a InitialPixmapPlacement=2 -a GlyphCache=1
It must be combinations of hardware that cause this, as this problem did not surface on another machine with a clean 11.2 install. This is the config of my machine with the issue:

OS: Linux 2.6.31.5-0.1-pae i686
System: openSUSE 11.2 (i586)
KDE: 4.3.1 (KDE 4.3.1) "release 6"
Video:Nvidia GeForce 9600 GT
Driver: x86-190.42

I hope this info might help someone else who might be experiencing the same problem. If anyone has any insight as to the cause of the issue, please post..