Reading Directory Opus Freezes after First Install prodded me into writing this just for information only because it also involves Windows Defender although I don't have the issue described.
I have a folder containing all my program installation files. The first time I navigate to this folder after a reboot, there is a significant delay before the icons are correctly displayed. I usually notice this when I navigate to this folder with the intention of immediately copying a new file into it. Pressing Ctrl-V (to copy) results in Opus "Not Responding" for a while. After a bit, the icons are properly displayed and the copy takes place.
The issue doesn't have anything to do with copying. Pressing Page Down results in the same delay.
This definitely occurs only when Windows Defender Real-time protection is on and icons are displayed (Details mode).
Those with a long memory may recall my reporting this behavior in the past as related to Microsoft Security Essentials.
It seems to me that I've had times during which the problem doesn't manifest itself during the Security Essentials era but I can't be sure at this point because I've sometimes used Security Essentials/Windows Defender and sometimes not.
For the moment, I'm solving the problem by turning off the display of icons in Details mode.
This is just for information. I'm not expecting anybody to fix anything.