I've had this issue for a while now, the Roku can run for days and then it will start stuttering and being laggy.. it has nothing to do with bandwidth because it will even have the audio stutter while navigating the menus of the Roku itself, without being in a channel.
It will remain like that indefinitely until I go and restart the device..
In my case I'm running 4660X - Roku Ultra with the OS at 9.2.0 build 4807-46, but I have had this issue since I bought the device last year... I've already exchanged it once and it does the same thing.
It's well ventilated, it's in my media center away from other devices and the media center has fan cooling for all of the components, the room stays at 72 degrees .. so it's not overheating or anything like that.
I have seen other people complaining about this same exact issue and I've yet to see any helpful responses that solve it.
It would be great to have it working smoothly, I never had this issue with my firestick, I went back to Roku for youtube.. I left firestick when Amazon and Google were in a fight
PS. I also had this issue on my Roku media stick.. I've changed tvs since then as well, so the tv is new, the HDMI cables are new..
This very much seems like either an OS issue or a hardware/design flaw.