also have this exact same issue. Nothing ever buffers on the box and all videos/netflix/disney+ all play fine once you actually get them working but it's an uphill stuggle to do so.
All apps are sluggish, icons take ages to load. Even once you are playing a video on say Disney and you want to press pause even that takes about 2/3 seconds to respond. Navigating around apps (disney/netflix) each click of the remote takes 1/2 seconds until it responds.
It's an odd one as like i said once you manage to get a video to play it plays fine, and in 4K with no buffering, it's just a UI issue. I have two streamboxes and 1GB (up and down), wifi 6 and generally ultra high speeds across the home.
I have one box on wifi and one streambox on ethernet and both are the exact same, with no discernable difference to hos sluggish the ui is. a bit slow and annoying i can deal with but it's close to the point where i dread having to switch it on as i know it's five minutes of waiting around just to fire up netflix so im close to cancelling the whole thing and jusrt going back to use my fire tv's.