I've been going through and adding Titles to my MKV footage through the main window, no problems. But I've been remuxing those MKV to MP4, and when I tried to add tags the same way, MP3Tag would freeze up, the larger the file, the worse it got.
Which is weird, because I was able to add a Title tag to a 6GB MP4 very quickly using the left sidebar, but this doesn't always seem to work. Sometimes it also hangs when using the left sidebar. But at least it shows the box "Writing tag data"