. . . . Is it possible to disable VST plugin scanning . . .
No.
Many programs, eg Cubase, Reaper etc, that use VST's rescan on startup, some, such as Musc Maker display a small info message while the scan is in progress.
An 'extended' scan time may be experienced depending on the number of VST's installed.
There are many DAW's that once you scan them they are built in a table or something so that when you want to use the plugin it's there. I shut it off so it doesn't take long to startup. Also the only time I rescan is when I add a new plugin.