Howard Miller

"I didn’t find any details in the task and system status log."

You're not going to get these messages and there be nothing in the logs. Make sure you are filtering the logs to find failed tasks. The logs will tell you (and/or us) what's going on.

Firstly, Benchmark is just a plugin somebody wrote and, while not to diminish their efforts, should not be considered authoritative. If your site is otherwise working ok then I wouldn't worry too much.

Having said that, I would say always start with caching. Moodle relies more and more on caches and the cache performance is critical. It's almost mandatory now to have a fast, Redis cache available on any serious Moodle site.

MTA = Mail Transport Agent

Normally you will setup Moodle to send mail as some user or other (e.g. noreply@mydomain.com) to your nearest mail server (e.g. mail.mydomain.com). THAT should bother about nasty things like DKIM, DMARC, SPF and so on. I'm really struggling with the notion that you would set that up in Moodle. If DKIM isn't set up properly in your mail server then Moodle is probably the least of your worries.

For those (like me) that were a bit surprised... you must be talking about https://docs.moodle.org/404/en/Mail_configuration#DKIM

DKIM is more often set up as part of the MTA configuration. I can't help thinking that's where it probably should be. Where does Moodle send your emails to? Doesn't that have a mail administrator that could deal with this?