Why does rendering on a PC take 10 seconds compared to 0.25 seconds in-camera?

I think you are missing the point. The camera gets raw data from the sensor, then writes a file to the flash card - in the process it creates that embedded JPEG you have been raving about. The OP has been talking about how quickly the camera creates that embedded JPEG - clearly it takes less than 1/4 second, because the camera can shoot 4 frames / second. Similarly, if you choose to shoot JPEG, then it creates those JPEG files at the same rate. So it has to be able to turn the raw data into JPEGs quite quickly.

Now do you get it?