@Kermit70 you have probably misunderstood this feature. Mp3tag library is not an actual music library, it is tag database to speed up file access on huge libraries and overcome memory issues when loading them into mp3tag at once.
About slow file access on your NAS, depending on filesystem, it may happen over time. For example btrfs and ntfs have defrag feature to fix file fragmentation (yes, fragmentation happens on network storage as well). Also how full are your drives? Most filesystems don't like if there is very little free space left (rule of thumb is no less than 10-15%).
Without exact knowledge about your HW setup, it is hard to tell, what can be the culprit and how to improve things.
Also, keep in mind that mp3 files themselves are quite small and instead of contigous sequential read/write, they need mostly lots of random read/write operations, which are way slower and more taxing on your NAS CPU. Also random disk access is slower than sequential one.
Hence the overall lower performance on reading/writing comparing to big mkv or iso file.
@ohrenkino I don't think that storing database on NAS would be very wise idea in this case, as you would loose all the performance benefits. When you store library on NAS as well, you would need to read all tag related data from NAS again, which pretty much defeats the purpose. Better to keep it on local fast SSD.