By chance I seem to have finally “solved” the annoying context menu lag on my TabletPC! :)
How was it done? I just installed the “Matroska Pack Full v1.1.1” (which contains many common audio and video codecs) and enabled its context menu extension. The most obvious (but not necessarily true) explanation is that installing Matroska’s context menu extension somehow fixed the context menu bug which was presumably introduced by upgrading my TabletPC to Windows XP SP2.
Now I’m really happy that my productivity is not restrained by this bug anymore :)