Speed/buffering issues

A 2160p or 4096x2160 4K stream on most providers will only eat anywhere from 15-25Mbps, depending on the provider and encoding used.  If it's a stream under 4K, the data required is substantially less.  About the worst data pigs out there are Peacock and HBO max, where you can get 'some' content in 8K, so you might top 50Mbps pulling a stream.  I purposely throttle streaming devices to avoid 4/8K streams to stay under the data cap.  You got 40Gb/day to play with for 1.2T/month.

If you are buffering, your internet connection may be picking up uncorrectable errors, maybe is dropping channels, and stalling out and breaking the connection to the provider.  What's your gateway say about it? 

Log into the gateway or modem, and look at the signal power both upstream/downstream, SNR, errors both correctable/uncorrectable and the error logs.  Perhaps you got something else going on that needs some work. https://forums.xfinity.com/conversations/your-home-network/internet-troubleshooting-tips/602dae4ac5375f08cde52ea0

Other WiFi radio issues?  Yeah, possible too.  Try to rule out the internet service first.  I don't have a newer Roku and didn't know there was a speed test on them, but 80Mbps is plenty.  Might be a hardware limitation of the Roku too.

Thanks for reaching out @user_d1d708! I'm so sorry you're having speed issues. When you test with your Roku are you doing anything else with your internet? Like downloading, uploading (Zoom meetings, Twitch Broadcasts) or streaming anything?