Something really cool about "reading good" that benefits people before they attempt to correct others. "We have an Ultra" was at the forefront of what I wrote. Your abilities to utilize this method and or your route to fix it might be different, and as with different models of different electronics, this is logically appropriate, actually it's something to be expected.
Got my Firestick a few days ago and I'm quite happy with it!
To start with, the CPU and Ram seem much better then the Roku Express. I tend to believe that this is the crux of the issue. The Firestick is far more responsive, menus change quickly as compared to the Roku. I like the Roku menu better, but it's slow compared to the Firestick. Had to Download VLC Media player from Amazon to play media from my media server on my router, but it works fine. Sling and VLC only 2 Apps I use for my TV system on the Firestick. There are many apps available like all smart TV's and more. You can even browse the Internet with it (like smart TV's do). I have watched a few Live programs and replays from Live and and no lag thus far!
So, my belief is that the processor is faster and no buffering required like my Android phone and PC. Problem resolved! The Roku is slower and buffers and video and audio streams can get out of sync if there is any glitch (OTA signal drop).
I have left comments and request for help with this issue for 2 years now, and there has been no response from anyone at Roku. This supports the theory that its a hardware problem that can't be fixed with software.