Apologies if this has been suggested earlier (had a quick search but couldn't find any reference to it) but it might be useful is when a message popped up in geoscape, the time factor thingie would automatically revert to slowest after clicking out - irrespective of accepting or cancelling whatever.
Case in point, the message that ground forces have shot down a UFO. You only have choice of 'deploy team' or 'cancel'. If your team is in need of reorganisation and you hit 'cancel', time reverts to whatever it was prior to the alert.
It's only an annoyance, but I've missed some opportunities because not quick enough to slow down time afterwards.