It's been common knowledge for a few years now, that page load speed is an important ranking factor. No one can argue that a slow website is more of an annoyance than anything and to Google an annoyed user will eventually equate to a user that will use another search engines.
So it's common sense that keeping your websites speedy is healthy for bottom line and Google's bottom line and that's why page speed as a ranking factor is so important.
Earlier this week (August 21, 2013) Matt Cutts of Google's Webspam Team answered a question via Google's Youtube channel (Google Webmaster Help) answered a question from a user concerning page speed and mobile websites (or the mobile version of your website) and whether page speed was more important for mobiles websites or not.
Matt did confirm in his video response that indeed page load time is indeed a ranking factor but the page speed ranking factor "applies across the board, it's not specific to mobile".
If you provide SEO services
If you're an SEO it's safe to explain to your clients the necessity for a responsive (mobile) site and why keeping their loading times optimal is highly important.
As Matt suggested in the video, Google is always looking for ways to improve their ability to determine fast or slow loading sites and so it's always a good idea to maintain a speedy site and check it's speed and overall performance on a regular basis.