Mp3tag (v2.79 & v2.79a) has been running very slowly on random file operations. Usually when I do a Tag to Filename custom action, it goes very quickly. Like in split seconds. For example, I typically open an album and adjust the tags to what I need for all songs in the album. Then, when done, I use a pretty standard action that changes the filename to my preferred format as follows:
IMHO it is a question of disc cache, simultaneous disc access by other programs, indexing software, players that update their library automatically, while you already catalogued files or move them to locations that are watched by these programs.
Or a Windows 10 update runs in the background...
So do not run players or indexers at the same time ... or other programs that require to be moved and fetched from the swap file.