On our server, where we play, there are often problems and difficulties with the appearance of mobs, as our server is quite popular and it plays 20+ players, there are very few mobs, only with a small number of players on the server you can see the appearance of mobs, pay attention to this and increase the spawn speed, we have BDS.

Also, for unknown reasons, the tick rate (TPS) becomes very low. Optimize the BDS, because it's just awful, and the provided core turns out to be just disgustingly unoptimized and somehow made something.
Server uses:
Windows dedicated server: Ryzen 9 5950X, 128 GB RAM, NVME SSD
Also, others have asked to add that your single-threaded performance is just laggy, and can't handle a huge number of players at once.