Hmm not sure about that. Lemme test with test files and I'll report my findings if my method is incorrect.
I mean do your editing to the individual files first, and then highlight them all, and then click the save button. Where it says <keep> or something similar, mp3tag should be smart enough to distinguish what has changed and what hasn't. If you highlighted the files, and then changed a field like track number, then all the saved files will have the same track number.
Edit: I think I know what you mean, woopididi. If you change the title, then you have to save individually. I was thinking in terms of album, where each album was the same. My apologies, you do have to save tags each time. But it should still be fast if you know what you're doing.
A faster solution may be to use mp3tag's filename -> tag feature, and change the tags that way. However, I believe it's faster to do individual editting.
Edit 2: There's also an autonumbering button that you can use. It numbers the tracks in order given that they're in the correct order.