I have two shares (A, B) on the same volume on a synology NAS. I'm have the two shares mounted in finder, but when copying or moving files it takes forever, even with SMB3 on. I can of course use Synology's file browser which takes seconds, but the issue is that I can't easly open and preview files - I would like to use Finder if possible or at least an app for Mac that would allow me to open the files.
Moving files from one share on a host to another share on the same host will always be faster when initiated from the host itself, and will always be slow when transferred from the host to a client (and back). Regardless of whether you use Finder, or
cp or Carbon Copy Cloner, if the data has to traverse the network, it will be orders of magnitude slower.
There are some technologies that allow a copy or duplicate command on the same share to be initiated by a client and performed server-side instead of copying the whole file down to the client and then back. I believe SMB supports this, but it it won't work across different shares, even if they're on the same host.
Long story short, your network will always be the bottleneck compared to the disk controller's native speed (~88MBps for Gigabit Ethernet vs. 200-600MBps for SATA). If you're running 100Base-T on your network instead of Gigabit, you'll be 10x slower than that (8MBps).
To (pre)view anything on the Mac, you'll always have to transfer it (or enough of it) to the Mac over the network. For large items, I'd recommend using the Synology's interface to move or copy items between the different shares.
From my experience, Carbon Copy Cloner as a rsync gui makes better use of the bandwidth available compared to copying using the Finder's drag and drop. If you don't want to use the NAS interface, that's the only way to maximize the transfer rate.
I will also suggest using Menumeters to measure your network speeds, and use a wired Ethernet connection instead of Wifi if this is applicable.