Hi Achim,
Could be a multi-threading issue, relating to unamb, which is currently not gc'ing enough useless threads. I haven't released the fixed unamb, because it revealed a ghc RTS bug. The RTS bug has been fixed but not pushed. Once there's an improved GHC, I can fix unamb, and see how reactive is doing. If there any bugs remaining, I expect they'll be much easier to resolve at that point.
Ouch. I played, or rather fooled, around a bit, continually resizing
the window. The first thing I noticed was that frames are indeed not
updated during stutters, the second that xmonad was quite helpful
killing xcompmgr blindly to reclaim at least parts of my X11 display.
In other words: Hell just broke loose.