Extreme Lag + Can't keep up

• Since we upgraded our multiplayer server to 20w17a we are having a lot of lag problems, so much so that we have to chop the blocks 2, 3 and even 4 times; and several of our redstone constructions have stopped working properly.
• It is impossible to play multiple players at once. Even traveling to chunks with nothing built and in daylight, we're still getting the message "[Server thread/WARN]: Can't keep up! Is the server overloaded?". It starts with about 2,000-5,000 ms when there is only one person, and goes up to +50,000 when there are three.
• We are aware that we have several constructions that generate many entities, but these don't seem to be the problem because it happens everywhere on the map, and before that we just had a bit of a lag as we approached them. 

I attach reports with a single player connected and then with two.