Much to my delighted surprise, it renders very smoothly in Firefox 45 on this rather resource-constrained machine, in a way that stuff like this almost never does.

I was also surprised at how it appeared to run at full frame rate on my aging iPhone 6

iPhone 6's are considered old these days? Yeesh. I thought they were still fairly powerful for a mobile device.

If they got it when it was originally released it's 2 years old now which is pretty old for any electronics in terms of where it'd sit on the performance curve.

It's in the top end of the performance spectrum still.

Great now I am sad because my phone is considered old :P

It sounds like GPU acceleration has been disabled or blacklisted. Maybe try another browser?

Same here. Still no hw accelerated 3d in Chromium for Linux I suppose.

Runs fine for me. Chrome 53, Arch, 270X with Radeon driver.