Optimize for Mobile
Responsive web design renders optimized versions of your website per your device via the miracle standard of HTML5 -- the "way forward" for web-based browsers (from 2012 forward) as established by Apple, Google, and Microsoft.
Quality mobile design will render your web page to your 4g cellular device in under 2.5s. Bad mobile design won't complete loading for 12s because it's waiting for some random unneeded script to load. You see this, and you avoid this. Don't be slow.
Google has made it very clear that mobile performance AND usability are key ranking factors. Why? Because if they send people to a page that takes 12 seconds to load, they'll stop using them.
What is Google doing about this? Providing all kinds of tools to webmasters to measure and test and troubleshoot and tweak. And eventually rewarded when a robust page is loading in under 1.5s.