Very slow Mail search
This has been going on for a while now. I am on 11.4 and tried many things. But did find a post that sorted in so thought I would put here for others, it means going to Activity Monitor and and then 'Force killing the process 'corespotlightd' will restore the search capabilities.' Thanks for whoever posted this (sorry do not have their details to hand). It solved the problem for me.
iMac 27″, macOS 11.4