possible cause
It may not be the case for you, but another possible cause is shared printers, shared files or mapped network drives that are not connected.
If you shared a printer from another computer on a network (or mapped a network driver/shared folder), every time you open a new file, the computer will look for that mapping. If you no longer have that other computer, other printer, other file, or the network didn't connect them at login, it will pause while it scans for it.
Small as it seems, I have had it lock up a computer for as much as 5 minutes every time a file is accessed.
It might not be your issue, but it's worth checking.