My server generally runs fine until a player enters a Nether portal, after which the lag increases (or maybe tick rate drops?) significantly.  Mobs and players become extremely choppy and block updates (e.g., mining/harvesting) take excessively long to process.  This lag persists in the overworld even when players return from the Nether.  All clients are on modern Windows 10 PCs and run fine before entering the Nether, so it's not likely a client problem.  The only way to clear the lag seems to be to restart the server.