It depends

rsync is fine, but to clarify a little further…

If you think you’ll stop the transfer and want it to resume (and some data might have changed), then yep, rsync is best.

But, if you’re just doing a 1-off bulk transfer in a single run, then you could use other tools like xcopy / scp or - if you’ve mounted the remote NAS at a local mount point - just plain old cp

The reason for that is that rsync has to work out what’s at the other end for each file, so it’s doing some back & forwards communications each time which as someone else pointed out can load the CPU and reduce throughput.

(From memory, I think Raspberry Pi don’t handle large transfers over scp well… I seem to recall a buffer gets saturated and the throughput drops off after a minute or so)

Also, on a local network, there’s probably no point in using encryption or compression options - esp. for photos / videos / music… you’re just loading the CPU again to work out that it can’t compress any further.