Responsive web design not about making sites for mobile devices, it’s about adapting layouts to viewport sizes. Responsive web design means that you don’t have separate mobile, tablet, and PC versions of your site. Slow responsive web design sites are bad for business. Your customers have web performance expectations no matter what device or network they are using.

Source: http://www.akamai.com/dl/akamai/slow_responsive_infographic_v7.pdf

See also:

• Structure of a Responsive Mobile Web Design [Infographic]
• How to Grow Your Mobile Presence [Infographic]
• WordPress Goes Mobile [Infographic]
• Should You Build a Mobile App or Mobile Website? [Infographic]