I don't know if this could really be called a "bug". More of an annoyance.
Operating System: Windows XP Professional (SP2)
MP3Tag Version: 2.39 (Also happened on 2.38)
Problem: If you select multiple mp3 files in Windows Explorer and right-click and choose the "Mp3tag" option from the right-click menu, the files take longer to load than if you drag and drop them into the Mp3tag window, or if you choose "Add Directory..." from the File menu.
When using the shell extension option in the right-click menu, the files seem to be loading one at a time into Mp3 tag instead of all at once. To give you an idea of the time difference, it took about 3.7 seconds to load 9 mp3 files using the shell extension. Using the drag-n-drop method, it took about half a second. This is not a huge time difference since we are only talking 9 files, but if you had 100 or more files, it would turn into a huge difference fairly fast. I have tested this on more than one XP machine and it does the same on all.
I am not a programmer so I don't know if this can even be fixed. I thought I would report it just in case.
Click the following link for a video that shows the problem. As you can see towards the end when I drag-n-drop the files, the process happens so fast you can't even see it happen. The video will restart automatically at the end. Once I drag-n-drop the files, that is the end.