Carbon Copy Cloner is my “go to” application for copying or syncing large number of files between Mac’s regardless of their location – it works reliably and fast across the internet as well. I’ve used it for more years than I care to remember, and it’s saved me countless times.
But when I copied all my files from a set of disk volumes across to the Synology I saw a big difference between using CCC and just using the finder. I easily reached 40MB/s using Finder copy, but less than 20Mb/s using CCC.
Then I found this explanation.
Eject the network volume in the Finder
Our first recommendation is to eject your network shares in the Finder, then run your task again. We have run several tests and positively identified an issue in which the Finder will make repeated and ceaseless access attempts to the items of a folder on your network share if you simply open the network volume in the Finder. This persists even after closing the window. This is a Finder bug, and it exists in both Mavericks and Yosemite. If you eject the network volume(s), then run your CCC backup tasks, CCC will mount the network volume privately such that it is not browseable in the Finder.