Recently I watched Matt Cutts interview about the importanve of site loading speed for ranking. So did you already observe it on your websites? And how do you estimate your website loading speed? I do it in this way.
Yes site loading time matters for better promotion on search engines. HIgh page loading time will lead to higher bounce rate and low returning visitors. Both are the significant factors for SEO things.
Yes, I been working on this. There are tools on the internet to help you with this. Faster is better.