this post from NHL sums it up.
The reason you're not seeing any difference in jpeg, is when shooting in this mode the output files are so small, that the camera internal memory can buffer the shots (even @ high speed) and parses them out to a slower flash. When shooting in raw or tiff however, the output files are now much larger (x3 to 5 times), and a single shot can fill the entire buffer hence the camera now requires time to empty it's buffer to the slower writing flash! This the reason behind why people complain about camera slowness in the raw/tiff mode.
In fact anyone can see the difference in flash speed right away in their camera when one goes to the video/movie mode and fill the camera buffer up (always limited in most cameras except for the S602) where the camera is locked up until the camera's buffer is emptied. There's less wait time between movie clips as flash speed rating increases. If you never use raw, tiff, or movie mode however, then you'll never be able to tell any difference in CF speed !!!