I recently tried building a few Linux VMs on a VMWare ESXi 5.0 host. All the virtual machines experienced very low network speed (<2Mbit/s) without any apparent reason.
After some digging I found the solution on http://docwiki.cisco.com/wiki/Disable_LRO:
Log into the ESXi host or its vCenter with vSphere Client.
Select the host > Configuration > Software:Advanced Settings.
Select Net and scroll down slightly more than half way.
Set the following parameters from 1 to 0:
Reboot the ESXi host to activate these changes.