If you use a Roku you can add their Speedtest Channel:
Speed Test | Roku Guide
You can also access the hidden Roku Bitrate menu via your Roku remote by pressing "Home" 5 times, then "Rewind" 3 times, then "Fast Forward" 2 times. This gives you a menu that lets you override the Automatic speed setting and choose a lower bitrate. You may also enable "Playback Debugging" to show your speed.