I'm not sure if you are able to help me, but I've scoured the net in attempts to find a solution, but have been unsuccessful.
In our WordPress MU setup, the wp-admin backend has an acceptable speed for the individual subsites, however when in the network site, performance is laggy. While it seems fine for now, I worry that as our sites grow, performance will suffer over time.
I currently run the webserver as a dedicated box with the MySQL database running on a seperate machine. Neither seem to be bottlenecked in terms of CPU by any stretch. The web server is running Ngnix on PHP7 (after a little work).
Ideas of things I can look to? I've played with caching but of course this only affects unauthenticated users (the vast majority of traffic). I have played with memory limits etc.
I have granted support access.