With Google adding page loading time as one of the factors for search engine rankings, webmasters have their hands full in optimizing their websites for better speed and performance. YSlow and Page Speed are essential tools for optimizing website performance. But then, one has to be using Mozilla Firefox browser and firebug extension in order to use YSlow and Page Speed plugins.
Enter GTmetrix, the online version of YSlow and Google Page Speed! It takes the URL as input and runs the tests in the background. The processing is pretty fast and results are accurate since it is using YSlow and Google Page speed in the backend.
The results are neatly presented with scores and the specifics breakdown for both YSlow and Google Page Speed. For ease-of-use, it also provides a bookmarklet. You can even compare your website with 4 other websites.
As for the downsides, GTmetrix comes with the default (V2) profile of YSlow. There is no option to change this as in the original plugin. It can be used for quick analysis of your website and not as a complete tool by itself.