Web page speed is important for a number of reasons. First, web page speed is a factor in the Google Algorithm for SERPs. Ideally looking for a page speed less than 2 seconds.
These 2 tools are useful to measure page speed, (and shows you how to improve the page load speed):http://developers.google.com/speed/pagespeed/insights/
and my favourite:http://gtmetrix.com/
Fast page load speed is also important from your client perspective. Slow page loads will frustrate users, and you will lose potential clients. This will 1) increase your bounce rate (bad in Google's eyes) plus 2) decrease the number of pages viewed and lose potential customers quickly.
If you are a Wordpress user, then the Iconic One Theme (free) is excellent - optimised for very fast page loads yet retaining a professional design.