intervalFactor :: !DisplaySpeed -> Real\r
intervalFactor Slowest = 1.0 / 0.2 // five times slower\r
intervalFactor Slower = 1.0 / 0.5 // two times slower\r
-intervalFactor Normal = 1.0 // two times faster\r
+intervalFactor Normal = 1.0 // normal speed\r
+intervalFactor Fast = 1.0 / 2.0 // two times faster\r
intervalFactor Faster = 1.0 / 5.0 // five times faster\r
intervalFactor Fastest = zero // no timer delay\r