Yes, I realize DirectX is the reason for the better performance. However, at the end of the day, why a game performs better on Windows matters very little to me. What matters is that it does.
Of course there is a lot going on. But, again, I don't care about that. What I care about is how they compare when rendering the page. That's what matters me because that's what will affect me and that's what will help me make the correct choice when purchasing a device for that purpose.
According to Sunspider, a MacBook 2015 and an iPhone 6S render javascript roughly the same. So enjoy your more portable device, while most people I'm sure would prefer a device they can have multiple applications and tabs open side-by-side. Regardless, enjoy your device, as I've said a million times. You can keep believe you're getting a desktop experience on a mobile OS, as I am sure millions of people are.