Looking to speed up my multisite wp-admin for subsites

I have about 40 active subsites on a shared server. I know I will need to move to a dedicated server soon (when I can afford it), but I am looking for ways to speed up the processing of my subsite users when they are logged into their dashboard. I have recently done a training with a few users logged in simultaneously for a training, and I noticed it was super slow for each of them in their dashboards.

I'm sure there have been some good threads on this topic. Can you please point me to some of these threads, or offer your tips here?

Thanks, Joel