Time passes very slowly in superflat

In a superflat world, time passes very slowly. It is not lag, my framerate is fine, but I do a debug profiling and my tick rate is always exactly 7.75 tps. (It should be 20 tps) Days last almost an hour, sun and moon jitter, smelting an item takes over 25 seconds, thrown items wait a few seconds to be ready to be picked up, entities move and fall very, very "laggily"...

I'm in a superflat world without structures, at layer 4, and nothing but slimes and nighttime mobs, no more than 70 entities. This is not lag, is a bug.

If i switch gamemode from creative or survival to spectator the tick rate automagically raises from 7.75 to 20, and if I switch gamemode back the tps drops again instantly to 7.75. This is hilarious.