Chances are you haven't noticed the library's speed improvement so far.
There are 2 requirements to use the library at full speed.
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It must be activated in Tools -> Library.
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Windows Defender as an antivirus application must not be active for Mp3Tag. For this you should define an exception for mp3tag.exe in the Defender settings.
Windows Defender - Speed of loading files in MP3Tag
There is also a different speed due to the filling of the cache if you have already loaded the files with Mp3Tag and have not restarted the computer but only brought it from standby. Therefore it could be that the different speeds that you have noticed so far only came about because of the temporarily filled cache and sometimes not filled cache due to a real restart.