The randomTickSpeed gamerule does not function how it should. Increasing the value decreases the tick speed to the point where the game runs extremely slowly.
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duplicates 1
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Duplicate of MC-76117
I checked and you're right. I got confused and thought that water and lava were also affected by randomTickSpeed. Although unless I missed a change in the snapshots fire doesn't seem to be affected in 1.16 while it is in 1.15.2 which is the original bug (or change?) I remember encountering. That's my bad for misreporting this.
Resolved as a duplicate of MC-76117.
The random tick speed gamerule changes the RANDOM Tick value, not the Game Tick value.
A laggier game is a side-effect of increasing the random tick speed, as the game has to random-tick more blocks, causing more block updates, ect.