I just had the same issue today and spoke with Cisco and they said "We will have to RMA this for you. We are aware of this issue". Out of the Box and back into the Box -R

Saravanan, We are achieving very good thoughput. See attached screen grab.Thank,R

I set WMM to required and upgrading to 7.4.100 and afterwards all of my IOS devices started working perfectly. very little latency and proper speedtests and download speeds were acheived.-R