High CPU usage after 1.18 upgrade

After updating the server to software version 1.18.0.2 CPU usage went through the roof when any player is logged in. It used to run 4 players simultaneously but now even with one player logged in it takes 99% of processor power (1 core only, it's on shared hosting). 

If I log in server usage is at 99% for a minute or two, then if I don't do anything it goes down to 70% and hovers around there. As I start mining etc. it goes up to 99% and blocks stop breaking. Reducing render distance to 12 (from 32 on 1.17.x) and tick distance to 6 (From 12 on 1.17.x) helped a bit - single-player login still takes the CPU to 99%, then after a minute, it drops to 30% on idle. When starting to play it climbs to about 70% and hovers around 60-90% with lag appearing very rarely. However, when the second player logged in it very quickly went to 99% and basically hung the server. Cant even shut it down.

The server is on Ubuntu 20.04, server version 1.18.0.2. Players connect from windows clients.

Moving the world to a local machine and running it there does not produce higher than usual processor loads. 

Please advise.