I think the cause of such issues is all down to how Sky/Comcast's streaming platform works. All the apps on Glass & Stream are hosted on a server, not on the device itself. This is the root cause of the general sluggishness of the interface and the reason apps can take up to 30 seconds to buffer up to even just HD quality.
The servers are clearly under an ever increasing load as more and more people sign up to the service so there's an awful lot of data being sent up and down to those servers from many, many connections. There's clearly a lack of cache space in the pucks as they simply don't cope well under any sort of pressure. Fast forwarding and rewinding for example, is such a slow & glitchy affair (even on a basic HD stream) that it's already showing how much it struggles.
With Netflix, they use higher bit-rates, Dolby Vision, HDR10, Atmos etc and therefore have to send a larger data stream back to the puck from the server. Also, with the actual app hosted on the server, that server has to communicate first with Netflix's servers in order to receive the stream of the programme you, the customer, has selected. This stream then has to be sent down to the puck. This all means the server is under a lot of pressure, particularly when there are potentially thousands of people trying to access Netflix from a puck or Glass TV at the same time.
The only real solution I've found is to just not use the puck for third party apps. I use the native TV apps or my Apple TV 4K box and have no issues whatsoever. Netflix has no buffering whatsoever and HD streams from the likes of the Channel 4 app play instantly in full quality.
Stream is absolutely fine for live TV, I've never had an issue with that (though it does take a few seconds for even those HD streams to reach full quality on a wired 500Mbmps ethernet connection) so that's what we use it for. Turn it on, select something to watch live and leave it - that works perfectly well. It's everything else that's problematic.
