Reaper always does a light VST scan on start-up. The first one after a reboot is a bit more thorough, subsequent start-up scans are faster (not sure why).

I think that it is done to make sure that the plugins listing in the .ini file are all available and to store their locations.

The "Clear cache/Rescan VST" function tries to load each plug-in too - so it takes a longer time.

Please Help.
Thanks !
I didn't find the right solution from the internet.
References:
-https://forum.cockos.com/showthread.php?t=86445
- Company Introduction Example