Depends on what you're doing with Mp3tag, and how your music library is organized into folders. If your library is organized into hierarchical folders, don't start Mp3tag up in the topmost folder. If you have large music folders, don't start up in a large folder full of music files.
In Tools > Options > General, there's an option to load (or not) Subdirectories. You might want to uncheck that. There's also an option to set a startup folder that you might want to use.
I always use the Windows Explorer context menu ("right-click menu") to start Mp3tag in an album folder, where there are generally less than a dozen or two files. It loads those files and starts in just a second or two.