Jumbo frames are Ethernet frames that are larger than 1500 bytes of payload data. Jumbo frames support up to 9000 bytes of payload.
It essentially means, you can stuff more data in a packet which involves less CPU/processing time and improves efficiency and performance. It is recommended that services requiring high throughput utilize jumbo frames (think storage, vMotion). To be able to use jumbo frames and an MTU of up to 9000, every device in the chain has to be configured for it. So, if you have two ESXi hosts connected to one switch, each host and the switch will need to have jumbo frames enabled and the MTU configured for each network interface being used.
Here is a link on configuring this on the ESXi side: VMware vSphere 5.1
You will need to look up your specific switch vendor/model to get the proper instructions (and avoid any downtime).
This would be my first guess for troubleshooting, if you configure the above properly your problem may go away :smileyhappy: if not, report back and we'll go from there.