I just upgraded to the newest version. Prior to that, my first save only took seconds. Now, it takes a minute for the first save, then, as I fill in other fields, the save is almost simulataneous. I usually do the song numbering first, save that, then put in the artist song titles, etc.
The first save is usually slower because the whole file has to be rewritten in order to make room for the ID3v2 tag. After that, ID3v2 is padded with 2 KB (IIRC) in MP3Tag so additional saves are faster. Only if you enter more data than available padding, the file has to be rewritten again.
However, I don't know why your previous version worked faster - it's the other way round here.