I would suggest that you're not using Mp3tag optimally if you need a feature like this. You should almost never need to load your entire library in Mp3tag. Do you really edit tags in all of your files? Do you even need to view all tags in all files to edit tags in, say, a single album?
Use the Windows Explorer context menu feature and right-click only the subfolder that you need to work on, select Mp3tag, and it will take just seconds to load.
Or, if you organize your library by throwing all files into just one or just a few extremely large folders, you should organize the library better so that it's easier to work with.
That may not be a bad idea, but only as an option. It wouldn't hurt anything.