This will run a simple Flash-based test to get an approximate speed for your computer... I plan to use something like this in future games to be able to adjust things automatically in game to make up for the lag that slower computers experience.
DON'T MOVE THE WINDOW OR SCROLL DURING THE TEST! Or it won't accurately read the framerate. Also your browser should be zoomed to 100%.
The application runs at a target framerate of 60fps. The number in the upper right is the percentage of 60fps you're getting (so, 100 is the best).
I suggest trying it on multiple browsers, or after adjusting settings (such as hardware acceleration in your browser and/or the Flash settings) to see where you get the consistently highest percentage - that will mean that configuration is the ideal one for Flash games in general (not just mine). Of course, it's actually a lot more complicated than that; a simple fps test doesn't really test your PC's capability, but it's a good rough estimate.
Technical details: 1750 muffins. Derpy sprite from Desktop Ponies.
After the test you can press 1 thru 5 to see all possible results.
Rankings are (in order): Rainboom, Dash, Applejack, Snails, Tom