freezes with certain mouseover pop-ups in gtk apps e.g. Firefox

Every gtk-based app I use shows severe lags (1-2s) when having mouse-over pop-ups. For example in firefox, holding the mouse on a tab, waiting for the pop-up showing the complete tab name, severe lag is endured.
I know this is rather vague and please let me know howto debug this furter, but so far:
1) Only in GTK apps.
2) Disabling kwin compositing does not matter

ProblemType: Bug
DistroRelease: Ubuntu 11.10
Package: kubuntu-desktop 1.239
ProcVersionSignature: Ubuntu 3.0.0-12.19-generic 3.0.4
Uname: Linux 3.0.0-12-generic x86_64
ApportVersion: 1.23-0ubuntu2
Architecture: amd64
Date: Thu Oct 6 03:22:43 2011
ProcEnviron:
 LANGUAGE=
 PATH=(custom, user)
 LANG=en_US.ISO-8859-1
 SHELL=/bin/bash
SourcePackage: kubuntu-meta
UpgradeStatus: Upgraded to oneiric on 2011-10-03 (2 days ago)