No other task has slow response. Mac Mail app is really slow, taking more than a minute to search for a sender. I had no such problem on Catalina on the same computer, a 2015 iMac. My MacBook Pro is still running Catalina, and searches in Mac Mail are speedy.
I have also discovered that when I search mail using Spotlight, the response is very fast, so I started using it for searches in Mail.
I have 429GB available on a 1.02 TB hard drive. I have 16 GBytes of memory. All apps I am now using were also on my computer when it was running Catalina, which ran problem free. The hard disk is running fine, and Disk Utility reports no problems when I run First Aid. No app is showing a high percentage of CPU. It is notable that when Mail is searching, CPU % is near 0 for all processes with the word mail in them.
I attached snapshots of Activity Monitor while mail is searching. of Activity Monitor while Mail was searching.