If you wish to set the speed of your connection for video streaming manually on the Roku video player, there is a code you can enter to access the speed override settings.
From the Home screen, press the Home button 5 times, the Rewind button 3 times, and the Fast Forward button 2 times.
A screen will appear where you can set the speed you desire.
You may want to start with 3.5Mbps selected, test out playback, then try a different setting for best results.
The Enable playback debugging option turns on a feature that will display the speed you are getting while playing video. This setting only works with streaming set to automatic.