Version: (using KDE 4.2.0) OS: Linux Installed from: Unlisted Binary Package Packages: Arch Linux official repositories I used ntpdate to correct the system time, and kwin composition was immediately disabled, because it was "too slow".
There is probably nothing we can do about it. The functionality uses Qt's QTime class to measure how long it takes to paint one frame. If you change your time settings, KWin cannot know it - this has to be fixed upstream at Qt Software. Even if I don't think this is really a bug. No "real" damage is done, you can just use alt+shift+f12 to enable compositing again.
I haven't looked at the code, but I guess it should be possible to add a sanity check if the time is negative or above some threshold value, then assume that something other than bad performance caused it. Another possibility might be to require an operation to be "too slow" a number of times before acting, and not kill the feature just because of one measurement indicating slow performance.
I agree with Robin, sanity checks should be added. Negative time should have no effect on KWin's checks but anything over +30 seconds or so can be safely ignored.
Also, when kwin disables compositing because it believes that it's being too slow (and most times it isn't) then it's not possible to enable compotition again through the GUI. It says that it cannot enable composite with the current configuration while it was working just a few seconds before.
*** Bug 187092 has been marked as a duplicate of this bug. ***
*** Bug 189403 has been marked as a duplicate of this bug. ***
*** Bug 197820 has been marked as a duplicate of this bug. ***
SVN commit 986962 by lmurray: Prevent KWin from automatically disabling compositing when the user changes the system time or timezone. BUG: 184607 M +7 -0 composite.cpp WebSVN link: http://websvn.kde.org/?view=rev&revision=986962
SVN commit 986966 by lmurray: Backport: Prevent KWin from automatically disabling compositing when the user changes the system time or timezone. CCBUG: 184607 M +7 -0 composite.cpp WebSVN link: http://websvn.kde.org/?view=rev&revision=986966
*** Bug 196328 has been marked as a duplicate of this bug. ***