After you enabled the DDoS filter to one of your ip's the filter will continious monitorring and filter your traffic. It will take an average of your traffic and knows whats normal and not. When you are downloading a test download file it is a long stream of large data. Depending your normal traffic, it will think it is abnormal and will block or slowdown the traffic.

Therefore you should not use a DDoS filterred ip to test the internet speed.