|Before running defrag, run disk cleanup and clean up all available check marks, then defrag. If important system files are badly fragmented, of many of your files are badly fragmented, then windows needs to stop and search for all of the fragments before it can open anything. Windows explorer especially, since it 'shows' you your files and before it can do that it needs to 'know' where everything actually is first. So if you have not run it in a while, do it.|
An infection is also a very real possibility. After running an antivirus program, whether it finds anything or not, also run Malwarebytes (free version) to look for infections many AV programs might miss (and keep it for occasional check up scans).
You have to be a little bit crazy to keep you from going insane.