speedtest.net results slow down

Hi,

We have CCR1072 which does BGP and close to 50K routes. Total downlink bandwidth at the peak is 5 Gig. When we run speedtest.net test for a 50Mbps queue customer the speed test jumps to 50 Mbps and then starts dropping and settles at 20Mbps or so. Have seen the same behavior with other shaped clients. For nonshaped clients, the behavior is the same. No packet loss or anything on the link. No bottlenecks etc. For normal downloads, the speed remains constant. Not doing QoS or anything anywhere. Tried fast track also but the situation is the same. Tried multiple speed test servers but no luck.
Did anyone face such issues?