I have been looking for answers and this was it along with realtek nic bonding in network settings. HOWEVER! It seems after the latest update for Unraid Connect (I wanted to give it a try), the slowness is gone Completely!
So if people are having sluggish web gui:
-Disable bonding in network settings if you have a realtek nic (make sure you have eth0 connected to your network or you will be disconnected).
-Update or remove Unraid Connect plug-in.
For comparison: it took about 20-30 seconds to load my docker tab before changes. It took 10-15 seconds when I disabled bonding. It is now around 1 second with current update of Unraid Connect plug-in.