I have a user on 10.8.1
When user opens an MXD his PC doesn't acknowledge anything happens for 5 minutes, then we'll see the splash screen. So issue is before the license check and data loading.
User can open arcmap and then the MXD without issue. User can utilize another PC and the MXDs behave accordingly.
No difference with user pulling the MXD from a server or their desktop.
Does the issue arise when you open a new MXD? If not, try opening your existing MXD file from within ArcMap.
Are there any broken links in the MXD? Are you currently logged in to ArcGIS Online / ArcGIS Enterprise from ArcMap?
It is in the processes before the splash screen, so prior to any loading of data.
Double click on an MXD and there's no sign you did anything except looking at task manager. After 5 minutes it will show the spash screen, validate the license, and load the data. Everything after the splash screen is normal.
Started recently, only changes could be windows patching.
Does it happen only with ArcMap, or any other non-Microsoft software as well?
If that doesn't work out, would you consider Undo a Windows Update ?