The switch I am using is built in to my ISP's router. It's quite a high-end router, so I don't expect poor performance. There is about 6 ft of CAT5e going to each machine from there. Running
iperf gave me 940 MBit/s, which is the theoretical maximum for Gigabit-Ethernet, so I don't think the actual network is an issue. When I transfer files from the same ZFS dataset via WebDAV, I get 110 MB/s.
So my assumption so far is that the SMB share is the limiting factor, either because of bad configuration (however, I didn't really change anything from the default settings) or a problem that everyone has, but no one cares about?