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

That will be true of virtually all interactive raw converters.  Any configuration change made that does not affect the demosaicing process will not cause that to be repeated... until the final command is given to write the image to an RGB formatted file, and then generally the entire process will be done from the beginning.  The reason for that is because initially, when the purpose is to generate a preview image, there are a number of "optimizations" used in the demosaicing algorithm that generate a (smaller) preview image and do it faster.  For the finished product the configuration will be different.