Part of me wants to just post the unhelpful response, which is "write less of it, because less code runs faster". That part won.
But then another part of me decided it would be friendlier to ask what exactly you mean - are you speaking of writing Universal Windows Apps that perform better on Mobile, or web apps that perform well on mobile web browsers as well as desktop browsers, or...? It's a really vague question we're struggling to get you a good answer to...