OK. From what you said it does sound like it "might" be your Roku.
One more thing to try: put you Roku in debug mode to see what you are actually getting.
While on the home screen enter the following:
Home 5x, FF 3x, Rewind 2x
Then choose "enable debug logging"
Then try and watch several streams and note at what speed the Roku reports.
Also, at a time when you are having problems on a computer go to http://speedtest.net/
and run their test. Run it several times and to different servers and note the results. Pay close attention to the latency.
Lastly, download and run "NameBench" from http://code.google.com/p/namebench/downloads/list
This will allow you to make sure that you are really using the best DNS for you.
To end the debugging just enter the sequence again and choose "disable debug logging"
This will give more info to work with.