Why does it take so long to read a MP4 directory (in v3.28 with i7/32Gb/2T SSD)? Also reading files via a M3U8 file is very slow.
Playlists are plain text files with references to files. So if the files are read not as fast as you expect then the playlists will not speed it up.
See this thread on a discussion about loading speeds:
How many files are you accessing? And how long is long?
Hello @MotleyG
I read your very interesting comment on the subject, I believe, that it is about the Playlist M3U8 command...
You say that the Mp3tag version 3.19 at the time was excellent with 25k songs, the performances are significantly faster than those of the previous versions.
I find your test excellent
I imagine that there would be no difference in speed between version 3.19 and today's version 3.28
Excellent question:
I wonder, do you think that the power of the computer can help to get the data faster, or does it not change anything?
Most important is the type of storage media you use.
If you use a modern internal NVMe M.2 SSD you can read your data much faster than (for example) from an attached USB hard drive or over LAN.
The CPU and enough RAM in your computer are also important, but not as much as a fast SSD.
The storage type and location is definitely one of the most important factors in improving access speed. It also helps to ensure the library function is enabled. My machine uses local storage with a SSD OS drive and traditional platter type HD for the media. It is managing a library of over 28k music files but is relatively old, at least according to Microsoft. Yet mp3tag can open the root folders and have the full 28k set open and ready in under 15 seconds.
Note that this opening speed can be impacted if a large number of files are modified outside of mp3tag. This means during the startup period the program has to fully read the metadata for all that have a newer file modification date than what mp3tag expected and update the library file. In my workflow, only mp3tag is used to modify tags or even update directory, folder, and filenames. The players and streaming programs just read.
Indeed, from my internal 2Tb SSD it goes quite fast, but from my connected USB hard drive the loading of the MP4 files goes terribly slow. That will be due to the reading of the metadata I understand, and that apparently cannot be faster. Thank you all 
Please compare:
SATA SSD up to 500 MByte per second, (~ 4000 Mbit per second)
USB3.0 5 to 30 Mbit per second.
So what happens if you copy the files from the USB drive first to the SSD and then load them?
The linked threads also suggested a number of further tuning steps like an exception in the Windows defender.
So please check those hints as well.
And there are some performance dates:
How does your setup compare with these?