There are a lot of ways to measure how quickly your site loads, but most of them aren't going to be reflective of how your users perceive your load time. Webslug is a tool that attempts to give you a read on the load time from request to fully-loaded. You can put in one URL or compare two URLs in a head-to-head battle.
It's nice to see a web-based tool to measure this stuff, no downloads required. I'm not sure how useful Webslug's data is, compared to the data you might get from other testing methods. There are aggregated numbers for different browsers and OS's, but those leaderboards might not mean much on their own. If you test Webslug, let us know whether you find it useful in the comments.