I have no idea whether this will help, for sure, but based on the possible sidetracking of your video stream by Roku's software, check the following:
Go to Settings > Privacy > advertising > limit ad tracking (check box to ON)
Again, Settings > Privacy > Smart TV experience > (uncheck use info from TV inputs; uncheck Enable auto notifications)
The rationale behind this is that these settings sidetrack your program material stream to provide Roku with saleable information. If the underpowered CPU in your TV isn't working on this junk, it might actually be able to keep up with your actual video stream.