This isn't my server, but my close friend's, and it's a problem that we're all experiencing and he's unable to submit a ticket at the moment. Currently, his server is running version 1.15.2 and up until very recently it's worked seamlessly. But today we encountered an issue, we aren't sure of the cause but it's resulting in extreme server-wide lag. Arrows are stuttering in the air, mobs move slower, redstone doesn't act as quickly, and for some odd reason zombie pigmen kept entering the nether portal, at a rate much considered unusually high.

Our confusion comes from the fact that the CPU usage is modestly low, and memory usage was only at 60% for the computer running it. The game is unbearable to play this way and we were unable to deduce the reason why this is happening. He installed the latest Windows updates, along with the latest version of Java to see if any of those would help resolve. Though it shouldn't need to be stated, we did restart the server several times but to no avail.