rsync tends to be a bit tougher on smaller CPUs. scp is generally better for those cases if you don't mind hashing afterwards to confirm everything made it without bit flips (in theory it shouldn't happen, but I've definitely had corruption during transfers like these). I personally prefer that method because you check files that are out of page cache and at rest on the filesystem.

If you want to get the advantage of SMB multi stream copies (in rsync*), look for LACP or other options. Not sure if Synology does that though.

I'm honestly surprised that SMB was faster here. I've never had luck getting it to perform well, so this is a bit of a shock.