We run several multi-sites that use the X Theme and came across a peculiar issue on almost all instances- all sub-site dashboards would load painfully slow but the main site would load lightning quick. We were using the same plugins and same theme on all sites so we were puzzled as to why only the sub-site dashboards would take up to 90 seconds to load.
After doing the typical trouble shooting steps of deactivating plugins, using incognito windows and activating one-by-one and even trying the default themes nothing seemed to work. We even considered placing blame on the SSL certificate but eventually ruled it out.
Turns out the solution was quite simple, it turned out to be a DNS cache issue. We finally figured it out after working on this issue for several hours on our office computers, I went home and logged into the multisite from a spare laptop and noticed that all sites on the multisite install loaded at a normal speed - the lag was gone. The following day I logged into my office computer and noticed there was still a 60+ second lag when accessing the dashboard, I closed all the browser windows and did a DNS flush on all of our office computers and the lag was gone.
I came across many discussions in the deep corners of the web that had a ton of different suggestions as to how to fix our specific issue only to find that in our situation it was a simple DNS cache issue. Maybe this post will save some of you some valuable time if you ever experience something similar.