I found my site was really slow after user login-in, so I use NewRelic to monitor its performance. Attached is the screenshot. But I don't know how to improve. Could you give me a few tips, please?
What is your current cache set up?
By default, Moodle caches to disk. This is fine if your disk is an actual local disk but on some VMs this is not the case and disk IO can be far too slow.
If in doubt configure a 'proper' cache server using memcached or Redis.
I'm suffering the same symptoms you are -- but ONLY with non editing teacher roles.
Student and Admin are blazing fast. NET's take 5 seconds to load.
I'm going to guess your on Moodle 3? There look to be some big time performance issues.
You might try upgrading to the newest Moodle and see if that helps. We can't upgrade at the moment. I wonder if this will help you.
Did you discover anything else since your original post?