GlobalSearch User Management is slow or unresponsive as a result of an exceeding large Active Directory and the entirety of it trying to be loaded at a single time.
There is a feature within User Management “User/Group Searching“, that when enabled changes the functionality of the User Management page so it doesn't load the entire AD on browsing. Instead, you can enter a specific user/group name and it will search for users/groups that contain that string.
In User Management open the Setting menu indicated by a gear icon ⚙
From the second tab “Options“ check the box for “Enable User/Group Searching“