I'm having a similar performance issue here: Win7 x64, DELL Vostro laptop, 20 months in use.
Have been using several progressive versions of mp3tag on this machine without problems.
Only the past few days (no Windows update took place, no new 3rd party software installed) the writing of mp3 file tag info started acting very strange. Track numbers and song titles would suddenly take forever to write to the file, freezing the app and/or concluding with an exception message "Failed/Cannot write to file <...>.mp3".
I tried starting mp3tag from the desktop shortcut instead of using the context menu, this "relieves" the situation for a short while but after a handful of tagging operation the misery starts again.
I tried disabling the virus scanner (McAfee) but this also made no difference.
Help...! My productivity at tagging files dropped to near zero!