I suspect this may be related to various redstone clocks (/fill clock and subtractive comparator feedback), but after playing a world for some time, I'll start getting massive hiccups where the game will not update anything for minutes at a time, often causing rollbacks.
The issue will not be resolved until the game is completely closed.
After closing, the launcher will pop up sometimes an hour afterward, as though just realizing the game has quit.

The obvious answer would be the /fill command executing 20 times per second causing issues.

Suggestion: Could we get a stable, adjustable speed clock, maybe as a block or circuit?