I have 3 FireSticks here. 2 of the latest regular sticks and one 4k stick.
Let me start with the overall device lag on the 4k that I don't experience on the others. Navigating the 4k stick is like sitting in rush hour traffic to drive 1 block. Everything is super laggy.
Moving on to buffering. Let me just state that I have no buffering issues on the regular sticks aside from the occasional fluke once in a blue moon. I know the internet isn't perfect. The 4k stick is a buffering nightmare. I can put the same movie or channel on both a regular and the 4k stick and I've got no issues on the regular stick while the 4k stick just swirls in random intervals.
The provider is obviously fine since I haven't had any issues on the regular sticks at all. I've had a couple of friends complain about lag on their 4ks which we all bought in the same week. They don't use IPTV so I can't compare that aspect. Is it just a bad batch or is this a common thing with the 4k sticks?
They are cheap, with cheap hardware. You get what you pay for.