It seems to work fine on my MacBook Air 2017 running the latest Mojave version, except when I select 10x where it just shows the first couple of frames and then the images freeze while the progress slider continues advancing.
It's possible the CPU and/or browser can't keep up with the video playback at that speed. I looked at the Activity Monitor on my 2014 MacBook Pro when playing the video 10x speed in Chrome - and the browser used 4 times or more CPU playing at faster rate. However, it did still play.
I also just tried in Firefox - which seemed to use a LOT more CPU than Chrome. Firefox topped out around 2 full CPU while Chrome was about 1 CPU. The videos in the first timestamp in the series also froze with the progress bar continuing. When it jumped to the next timestamp in the list - the videos were playing fine. So probably a combination of video encoding quality and browser/CPU usage slowing things down.
Jeff