When using a software package that uses a Transport Driver Interface (TDI) driver you may notice slow network performance. Some antivirus programs like Trend Micro use TDI drivers.
You can download a hotfix for this problem here kb2493361
As some of you may have noticed this issue has happend before but for some reason they don’t put the fix in the SBS 2011 release, so if you have this particular problem install the hotfix.
UPDATE: The update is included with Windows Server 2008 R2 Service Pack 1, so if you do not have installed SP1 run that in stead of just downloading this update.