For people who don't have much RAM but have a USB 3.0 ultra fast SSD USB drive, latency goes from 14.1 to <0.01 ms, speed go from 50MB/s to 250MB/s R and W... That is a substantial increase in speed.
it is the latency that matters the most here though. Of course, if you have 1GB + RAM, ready boost won't help much except in extreme cases