I would like to add few additional insights, although not specifically related to the "Defender" thing.
Still using one machine here, but noticed considerable decrease in performance and stability after several Windows 10 updates, which occurred around end of March.
It was mostly related to 3ds max instability, frequent freezes which eventually influenced Corona experience.
Made investigation as read a post somewhere that the following windows 10 updates "B4535996, KB4540673 and KB4551762" can cause some serious trouble.
Happened to have had installed the latter and after removing it - back to normal.
Completely support the idea that we need to organize a windows 10/3ds max file or note where to put most common software related stuff that might be the real culprit behind some of the issues people are experiencing.
I am personally working from a network and almost every single file becomes quite slow at certain point. Trying to avoid keeping multiple 3ds max instances opened as well, but... avoiding is not a solution. :)
Still using 3ds max 2018.3 but it is bit sluggish in UI, bit slow and more unstable than 3ds max 2018. Tempted to try most recent versions, but really do not want to go down the rabbit hole.