Why does it take so long to write ID3v2 tags?
The ID3v2 tag is placed at the beginning of an audio file. If there isn't any ID3v2 tag in a file, there is also no space reserved for it. That is why a new temporary file needs to be created which will store the tag. Then, the audio data from the old file needs to be appended to the newly created one. This procedure will stress the hard disk, resulting in a long operating time. If you want to edit an existing ID3v2 tag, the editing procedure will take less.
Deleting an ID3v2 tag will also take long, as the audio data from the tagged file needs to be copied to a new file.
If you don't mind the ID3v1 30 characters limitations of the Artist, Album, etc. fields, you can use the ID3v1 tag type instead. This is placed at the end of the file and with its 128 bytes size it is very quickly written.
Mp3tag also supports the APEv2 tag for MP3 files, which can be written, edited and read quickly. Its only disadvantage is that the only player capabile of dealing with it is foobar2000.