Saving tags to files sometimes take longer then usual on some files. While writing tags to those files the UI gets unresponsive and doesn't react until this task is done.
Looking at the log it seems like "file.update:497" takes about half a second on those files. (Line 347 and onwards)

While I don’t think that it’s a problem that updating those files take some time (Picard/Mutagen might have to move some data around in the mp3), I do think that the UI should still be responsive during that time.