When you drag some files to another software, interface of mp3tag is blocked until all files are loaded in the secondary app.
This scenario is really frustrating when you do it a lot, in my case I do it frequently with Audacity audio editor in background - f.ex. I edit tags in mp3tag, file after file, then I drag some files to compare in Audacity, and then I need to wait some (or a lot of) time until I can use application again. If this might be somehow fixed, that'll allow edit next files independently of the Audacity's loading progress (or any other app).
I'm talking here about Windows version of the app. I have a strong hunch that it's a Windows' UI libraries "by design" behaviour, but I'm sure there is a workaround for this