When doing some batch updates with files located on a NAS, it is awfully slow (5 minutes for 14 Tracks).
When doing it on a single file, it is obviously slower than on a local file (this is expected), but acceptable 1 second.
This is quite long, but also depends on your connection to the NAS and the type of file and changes you're performing.
With some file formats, Mp3tag needs to create temporary files which are then re-copied on change. This often happens when you add cover art. If the next time you just change a character, it should then be faster.
Can you give some additional details on your system, the NAS and it's connection to your Mac, and the type of files you're working with?
I've tried to reproduce this here on a DS220+ without success — performance is way better with ALAC files of comparable sizes. I'm mounting the network drive via SMB.