I am having this problem on my private Minecraft server, where I play with 3 others.
It's especially noticeable at my slime farm which I built and where not even one slime had spawned after roughly two hours of AFKing (I actually got one single slime spawning in this chunk before I built the slime farm by /killing all zombies, skeletons, pillagers (291!) and creepers, so I know, that it's an actual slime chunk).
But also on the surface I get quite low spawn rates while on difficulty hard.
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@violine1101 I think, that's not the whole issue, maybe it's both, because as I said, the spawn rate of basically all hostile mobs is pretty low.