Could be that your Ethernet cord is crappy, you're using the cable modem provided by your cable company, or you have the bare-bones Internet plan.
Even if you live in a Bill Gates mansion, hard-wire your computer and Roku (or Apple TV, Chromecast, etc.) Don't use the cable company's modem; buy your own. My Roku and computer are hard-wired; and my modem has dual wi-fi so I can use my smartphone as well.
does fiber optics use a modem or is this an ignorant question?