Doom fast moving crushers start desyncing from identical crusher sectors the moment when they start crushing something.
Slow moving crushers slow down when crushing something but fast ones should keep their constant speed. The deviation is small at first but it grows on every cycle during the time there's something to crush. It's as if the crusher ceiling doesn't go as far down as it should.
Example case: Doom.wad E2M2. The hallway has five crushers, the three middle ones have the same floor and ceiling heights and they're activated at the same time. Trigger them in -nomonsters game, use god mode and walk under one for a minute. (no pun intended
) Compare the results.
Note: only the walk over line 300 mu to the north from the red door will trigger the e2m2 fast crushers. The lines in between the crushers trigger slow moving crushers. Once triggered as fast moving crushers they'll keep as fast even if retriggered from slow moving crusher line. And vice versa.