ESXi 5 network performance is slow

We just did a fresh install of ESXi 5 on a host that was running ESX 4 before. Nothing has changed hardware wise. After the upgrade network performance is much slower. Even copying a big file from one VM to another VM within same virtual switch is slower compared to other hosts that are running ESX 4.

Network cards are auto-negotiating at 1Gbps as were on ESX 4 prior to upgrade. All settings are default and I haven't played with Advanced Settings at all.

Before opening a case with vmware, wanted to know if I am missing something or if others have experienced similar issues and found a fix?

Not sure if following would relate to your network or not, but we have had this issue as well. What turned out to be cause was network adapter load sharing and failover settings. vSwitch for VMs is using 2 network cards spread across multiple switches. We were doing load sharing (outgoing) between them and obviously internally traffic would come through only 1 nic.

After our upgrade to ESX5i we have had slow performance with this setting. We moved one of the NIC as failvoer NIC instead of load sharing and that fixed our issue.