Cisco WAN :: Slow Speed Over Comcast 50 Mb Using 2951

We have 50 Mb Comcast cable conencted to 2951.  There is another conenction to AT&T 20 Mb circuit which goes thru' an ASA 5510.  Path to Internet is as below. [code]
 
As long as Comcast is up, 2951 sends Internet traffic out to Comcast and uses AT&T via ASA for backup.When traffic goes over Comcast, users complain about slow speed out to Internet.  If we force traffic to AT&T via ASA, speed issue goes away.
 
We don't see any issue on 2951 router in terms of CPU or memory util.WHat can cause slow speed despite the fact that router resources are not maxed out and Comcast circuit has 150% more capacity than AT&T?