Buffering with excellent internet connection

If you don't have sporadic speed slowdowns and/or jitter/latency, and your wireless connection isn't weak or interfered with by others, I'd be thinking it's further upstream. Maybe your ISP throttles certain content (some are, or have been known for that). Could be the provider themself is overburdened with traffic?

I wonder how much of streaming "apps" go through Roku, and whether Roku could be the bottleneck. For example, the new antenna tv experience is very dependent upon the internet connection to the "guide info." Mine has been very slow at times. The way the interface is written (now), it makes the entire experience slow (won't display the picture until the guide info arrives.). I have 300 down/30 up. I'm using 5g wireless. My laptop (connected similarly) gets great stats from the test sites. But, that guide is like I'm on a 9600 baud modem. Something more in that guide's domain is the slowdown.

Does your buffering happen with any Roku streaming app? I would test that out. And/or, do a factory reset. During the initial setup process, tell it you don't have internet access (I've gotten the impression it will load whatever version it came with. But, that might be the Roku TVs only.). What I would try to accomplish (with a Roku TV anyway, and I'm assuming you can do it with the external "Express" device) is to get back to a prior update, then configure the network for the device, and see if the streaming works better until the periodic update process runs. If it gets worse, then you'd know it was Roku's update. (It's hurt a lot of people in a lot of different ways.).

I'd be trying to narrow it down a few different ways.