It takes some work to figure out where the problem is. Needless to say, with FiOS speeds, we should never have problems.....but sometimes we do.
I struggled for a long time with problems and eventually noticed I also had problems with the History channel and a few others. I called Verizon, but they were hesitant to send out a tech. The usual fix is to send out a new box or reboot the system, none of which worked. Eventually, they sent out a tech to see what was going on.
Shortly after opening the ONT box, he noticed a red flashing 'video' indication which explained the channel problems. Signal was very weak for video. He had to call a line technician to physically check the fiber upstream of the house. Problems were found and fixed upstream.
I then had a solid 50Mbps, and have had that ever since. No more Verizon video problems ever since.
A few nights ago, I had the same HDX problem, but immediately ran a speed test and was running 50/25 as it should. So, problem was not with FiOS.
Rebooted Roku3 and system worked fine. I have read of some memory leaks in the Roku boxes, so now I unplud the Roku when not in use so I don't have further issues from that item.
Good luck findiing the problem. Check every item in the line. If you are not showing speed issues from a wired pc, it is not likely a Verizon issue.