For weeks, I kept having Apple Mail slow to a crawl, such that I could only type about one character per second, if at all. Then it would go back to normal after about a minute. It turned out this was when the Move if SpamSieve Spam AppleScript was running. I disabled that rule, and no more slowdowns. Of course no SpamSieve filtering either.
Also, for a while now, email identified as spam was not being moved to the spam/junk folder, it stayed in the in box. I went through a troubleshooting page and tried four or five things (that was last year sometime) to no avail.
I’m on 10.15.7, running 2 G Suite and one iCloud account. Admittedly I am a terrible email filer (currently 8000 messages in the 3 inboxes, which I clear out every year or two). SpamSieve worked great for many years (at least 14) before this. Any suggestions?