When files in a folder are changed Windows will notify MC about that. And MC will refresh the file or entire folder if needed. but normally entire folder is not needed. But A rescan should not use a lot of CPU, (Windows even cache it.)
I have no idea how it can use lots of CPU for you, because it can be many many different thing that causes it to be slow.
I'm not able to reproduce it, not even on my old slow machine.
You can try to disable things that are running in the background like not showing icons, not showing overlay icons, make sure you do not view in thumbnail mode. (since thumbnails are fetched in background) , Only show standard columns (name,ext,size,date,attribute)
If there are a lot of .exe / .dll files in the folder the AntiVirus software might hook in the scan the file before the program can get the program icon.