So I too had jitter in the form of a hickup once every 5-10 seconds of Youtube video play at 720p on full screen while running a Raspberry pi 3 B+. Though in saying that, it did kinda mostly flow fine when in the smaller window. I am running an HDMI cable to an old TV monitor that really probably only does 720p but whatever.
The real trick was that I replaced my micro SD 32 gig chip with a better one. Yep, I know that you think that your class 10 chip is good enough and that's what I thought, but nowadays not all class 10 micro SD chips are the same, I bought a Sandisk Extreme plus with a V30 written on it. This simply means that it will move data in and out of the chip at more than 3 times the speed of your cheap class 10 micro SD card... which apparently is important!?
I actually won't claim to know why this is so important since I kinda thought that the internet content was simply heading straight into the processor and not even touching my Micro SD card, but hey, what do I know. Anyways, I also noted that my startup time for My RPi 3 B+ went from a 30 second time to get to the desktop to a 17 second time, I think that this is a tell-tale that things were sluggish with the old micro SD card that I had.
Anyways, the other improvements that I did were to update my Raspbian to whatever was current, maybe that wasn't really important, and to get a power supply that didn't always have a lightning bolt showing up on the top right hand of my screen... probably not critical either.
Either way I can now watch Youtube MMA action type Videos at full screen at 720p on Chromium without hickups. No re-writing of code needed here. Hope this helps.