The alert message text could certainly be worded better.  This means that your link has negotiated a lower speed than it's maximum, for some reason.  Often this is because of a bad cable.  The other thing I'll note is that we have had some bugs in the past where the 1g NICs would be the culprit but report/alert as if the 10g NIC was the problem.  So in your case, this might actually be a 1g link issue.  In any event, check your links to see if one of them is reporting slow.  You can just run "ifconfig" from the cli on the node referenced in the alert.  Probably wouldn't be a bad idea to open a Support ticket as well....
Hi,     This is a strange message that we saw on our isilon cluster, what does this actually mean?  Normally i have seen this link going down or it is flapping but this new message is strange, Can anyone assist?
10-Gigabit Ethernet link 10gige-1 (cxgb1) running below capacity