First of all, let's try to verify that OS X is using SMB2 for your shared drive.
Can you connect via Finder to your NAS share drive? Then open a
Terminal (type terminal in Launchpad) and execute the command
mount without any parameters, like this:
This should output you several lines, one of which should be about your shared drive and how OS X is opening it (using SMB, AFP, NFS, etc.). Can you confirm that OS X is mounting it as SMB (or SMB2 if it shows that level of information)?
This is to confirm which protocol is effectively used.
Once the protocol is confirmed, try via the command line to copy a file > 250 MB. For this use the command
rsync to show the progress, here is an example assuming you have in your home directory a big file named
big_file.ext and the shared drive on your NAS is called
rsync --progress big_file.ext /Volumes/share
Do you have the same behaviour? Can you describe it better now?
Finally, it seems you are not alone and people are proposing workarounds while Apple is preparing (hopefully) a fix: https://discussions.apple.com/thread/5467191
In addition, try updating your Synology OS to the latest version. DSM 4.3-3810 has the following changelog entry: "Enhanced the compatibility of SMB 2 with Mac OS X 10.9"