Speed is an important metric, but only up to a point. Once a browser renders a page faster than a user can interact with it any further improvement has very little impact. Any phone released in the past year or two meets that criteria for a well constructed page (obviously a badly written site is always going to perform badly, even on the best hardware.)
So, while it may be true that an Android phone is 5x slower than a new iPhone, that doesn't make a difference to the end user. They see the website working well on both devices. At that point it starts being about features, so iOS's lack of support for WebRTC, Beacons, Gamepads, Fetch, etc means the experience is worse. Not that Android supports everything, but if you compare iOS Safari to Chrome on Android[1] it's very clear that Safari is behind, even if it's faster.
[1] http://caniuse.com/#compare=ios_saf+10,and_chr+51