When you set the randomTickSpeed to a high number like 999999 the game visually runs, but auctually stops.

When you set the randomTickSpeed gamerule to a really high number like 9999999, the game stops. You can still move around and break block, But the blocks you break doesn't do anything. For example, if you break the bottom of cacti, the top part doesn't break. You can't interact with entities either. You can't punch mobs, when you drop an item, it gets removed from your inventory but it doesn't show up in the world. It's basicly the game actually stops, but runs visually. When you try to quit the game it just says: "Saving World" and i think freezes. It doesn't crash, though. It just stays like that. The only way to actually to close the game is by task manager (Except I think MacOS. Or I could be wrong.). But when you do close it by task manager, it says "Game Crashed" in the Minecraft Launcher just like when it says in other crashes. When you try to join back though, everything you did under the effect gets removed so it doesn't corrupt any worlds. I think you've already got a crash report for this, but i reported it anyways. I tested this bug in Minecraft Snapshot 19w46b (Where I actually discovered the bug) , 1.15 Pre-release 1 and 1.14.4 (Which I did for the report.). I hope my report was useful and that was all it.

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