Perhaps the reason for that is it’s been tested on Safari on an iPhone but not on Firefox on a fast desktop processor because the method used to animate is something fast in 1 browser but not in the other. Perhaps your comment didn’t consider that Apple’s marketing team only cater to their existing core demographic most likely already using an iPhone and not interested in the product at hand. But most of all, you didn’t bother to consider the software stack that had a huge impact on performance. And neither did Apple.
Then your "beefy" development machine is less powerful than an iPhone, since I tested it on that, and it works fine.
When I get to a full computer, I'll try it on that. I wonder if the ones I have are "beefy" enough.