@violine1101 is there any news on your progress with this bug? It's really ruining the server I play on. If it helps I can send the world files of an affected world.
I have not noticed any correlation with slime chunks either. However I have noticed that when it occurs it always seems to occur upon login on a multiplayer server.
This comment noticed the same thing: https://bugs.mojang.com/browse/MC-142134?focusedCommentId=541020&page=com.atlassian.jira.plugin.system.issuetabpanels%3Acomment-tabpanel#comment-541020
We have stupidly high processor usage as well. We started the world in one of the snapshots (not sure which one) and have upgraded though every snapshot and pre-release since.
We have a couple of guys on our server affected by this, it's a major problem for them and it means they can't safely traverse the nether.