Rsync Music Library with PcP USB drive incredibly slow

Hi, Don't know if this has been already treated somewhere in the forum I couldn't find it.

I have pCP 8.0 and LMS 8.2 and I have and USB drive WS 4TB attached to the Raspberry Pi4. I'm using Rsync to sync my Mac's Itunes library folder with the USB drive attached to the Pi4. And I found it insanely slow; I'm talking of more than 10 hours to sync a 25,000 items library.
Now, before attaching this very same drive to the RPi4, I had it watched to the router, and it took me from just a few minutes to max 1 hour, when I had many files to transfer.
Now with the usb attached to the RPI, it takes hours even just to check if files have changed at all; so even without any file transferred. The usb drive is exFat formatted. I had to move it from the router to the RPi, because the router stopped recognizing it, but that's another story. and in any case I understand that the USB directly attached to the RPi is the most suggested solution for pcP. So I thought I would have had less problem in any case, but unfortunately rsync is extremely slow, t the point that I prefer to plus it on the Mac to do the sync.

But I don't see any reason why it should be so slow; does someone have any idea?

Best regards