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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I tested it on 2 FAST and GOOD computers.
1 MacBook Air, 1 windows
My coputers can handle it.

Client isn't affected by randomTicks, so it runs fine. Which is why you can run around etc
The server hasn't stopped, but it is just taking a huge amount of time to process all those randomTicks
This is not a bug
As Violine said: if your computer cant handle it, dont set it to such a high value
Some people would call this a feature, as they are able to slow the game down easily with ingame comamnds
Thank you for responding, but when you discover it it feels like a bug. Whatever, at least I know now what it is. Keep the good work!
Don't set randomTickSpeed to such a high value if your computer can't handle it.