There is a huge amount of lag while running a vanilla server with a few data packs (But it does not help when you turn off the data packs). What you see in game is mob lag and sometimes lag spikes where it puts you back a few seconds. We have also had the issue, where the game was stuck totally and the time did not proceed, plus blocks could barely be broken and not picked up , plus the nether was inaccessable. So we had to recover a backup, but the lag still started up again. 

I could not see anything in the console. We have absoluetly no idea how to fix this. The ONLY thing how we fixed it, was downgrading the server to 1.15.2. This absolutly removed the entire lag and the game was running smoothly.

The problem is that I dont know how to reprocude this issue, since its a server side lag, which corrupts the world. (Server side in meaning it's got something to do with the version.)