VMotion falling at 10% can have a lot of causes but I have not found on the list the problem I had encountered. My machines were not migrated and after some time the procedure was giving „Invalid fault” error.
I run through a usual list of suspected problems: mgmt services were restarted, vmkernel ping checked as well as name resolution.
The problem was easy to solve but difficult to spot: there was some problem with cable / interhost connection because a NIC for VMotion on one of host did not autonegotiate correctly and set the speed at 10 Mbps. Which was consistent with what I had observer before the first „Invalid fault” error occurred – one VM was had been migrated extremly slow.
After both NIC were set manualy to the correct speed, VMotion started working fine.