I also know this because when I use a computer, iPad, or iPhone to access the same channel (YouTubeTV) in the same room as the Roku the connection is clear, HD quality and zero pixelation,. This clearly indicates there is something wrong with the Roku itself.
Outside throttling is still a possibility. It's meaningless to compare using different devices, as almost all major streaming providers utilize different servers (often located in different geographical locations) for different platforms. So your iOS devices and computer are likely not streaming from the same site as the Roku. I'm not saying it's likely, but I wouldn't summarily dismiss the possibility.
I have a 3920 Premiere, although it's likely the least used device in my home. It was released in Sep 2018 (7th generation), so while it's not terribly old, it has been out-gunned by the two generations that have arrived since then.
I'd say it's most significant potential issue with the 3920 is it only being a single band WiFi. Since it's stuck on the 2.4 GHz, band, which has the most interference, it can't benefit from the relative quietness of the 5 GHz band. My Ultra 4800 sits less than three feet from my closest WiFi access point, and the performance difference between the two bands is enormous. My Internet is near Gigabit speed. My 4800 will report 225 Mbps on 5 GHz, but can't even reach 100 Mbps on 2.4 GHz. Some content streams fine on 5 GHz, while the same content will on occasion choke on 2.4 GHz. I can't emphasize enough how much better the signal quality is on 5 GHz.
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