I have been having similar problems. It started about a week ago after an update session. My machine takes an age to start up and log in. As soon as I start an application everything stops. CPU is at 100%, delay time up to 30 secs. I scanned for infections using Malwarebytes. Then removed it and also removed McAfee that is was using as antivirus. I also scanned with registry recycler. I restarted defender. Still the same problem.I have two harddrives. My system drive is 500Gb and has 420 Gb free and I have a storage drive which is 2Tb and has 540Gb free. Eventually I found that most other programs I tried start normally, which is pretty fast but the one that is causing the problem is Internet explorer. I am going to try to uninstall it and reinstall and see if that makes a difference. I wonder if an update to internet explorer has caused the issue. I will keep you informed.
Any more suggestions will be gratefully received. Cheers for now.