Superslow tagging on mp4 files

I love almost everything about mp3tag. When I first starting tagging mp3 files I used Media Monkey and just didn't like it. Then I discovered mp3tag and found it easier to use, fast (on mp3 files) and I preferred the layout.
There are two things though that drive me nuts:
The overly bright colour scheme and most importantly the so slow mp4 tagging. It can take up to five minutes to tag one film adding just a title and date tag.
I have done a test tagging twenty films with the latest versions of both mp3tag and media monkey. Mp3tag took just over thirty minutes and Media Monkey took less than one minute.
Please tell me I am doing something wrong, perhaps there is a setting I need to change. I still don't like media monkey, so if there is a way to fix this I would be a very happy bunny.
I understand you are likely to want a lot more info so please fire away with your questions if you think you can help.

It's possible that the files you're working with have a lot of chapters and Mp3tag is (currently) not very efficient when writing such files (reading is fine).

If you don't need the chapters, you can try removing them via the right-click context menu Tools → Strip MP4 chapters. Try this on a test file first to make sure it's not due to a slow drive or network share.

Your vote regarding a dark color scheme is counted.

Thanks for the fast reply Florian. None of my mp4 files have chapters, but just in case I am more of an idiot than I realise I tried what you suggested. It took at least as long to running that command as it does to tag the files.
I had considered the drive/network issue, which is why I tested the previously mentioned 20 mp4 files on the same nas drive. I made two copies of each file and added two tags to each file in one set of copies using mp3tag. Then I did the same with the other set of files using MM. The difference in speed between the two programs was enormous. I did the same again today with twenty other files, but got the same result.
Could there be some settings I might have changed in mp3tag that could make it run slow?

Have you checked with MP3tag whether the modification you made with MM has actually been written to the files?
It could also be that MM only updated its database but did not re-write the files.

Yes I checked and mp3tag show the tags I added with MM.