Hello,
maybe I'm now a little bit of arrogant, but the problem is with one of your network card. But I cannot say its the NIC in your Workstation or the Broadcom from DELL.
Why I believe this - see an old entry from me with VMware Converter and (http://communities.vmware.com/thread/330060) and Broadcom NIC.
And a couple of minutes ago I tried to reproduce your Problem with my slower ESXi Server (32GB RAM, E3-1220 CPU) and my actual File-Server and my old, maximum of 33MByte/Sec transmitting Server from Year 2010 (should be disposed, but still works...).
Here my Results, see attached Screenshot:
Actual File-Server --> ESXi 5.1: 60MByte/sec, Peak 88MByte/Sec
Old File-Server --> ESXi 5.1: 27MByte/sec, Peak 33MByte/Sec
Acutal File-Server --> Old File-Server / Windows 2012 --> Windows 2003: 33MByte/sec according to Task Manager.
Means - my old File-Server is the slowest Part into this environment.
NICs:
ESXi 5.1: Intel 82574L
Fileserver 2013: Intel 82574L
Fileserver 2010: Broadcom NetXtreme
My suggestion: If you have an certified Intel NIC around to put into your ESXi Servers (see VMware HCL), test with it.
As I just have Intel or Braodcom NICs and no RTL or Atheros Cheap-NICs, I can just assume that the NIC is your Issue.
Regards,
Josip.