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

Rendering is the process of generating an image from a model (file), so when you talk about "rendering the RAW data to get the embedded JPEG", this is incorrect.  The camera NEVER uses anything other than this embedded jpeg to render the image on the rear LCD and that is why it is so quick.

Your second paragraph also suffers the same confusion. It is simply not true to say that the camera renders raw data. It does not. It only ever renders the jpeg embedded into the raw data at the time it is written. When rendering, the camera never accesses the raw data.

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Alistair Owens
http://www.alistairowensphotography.com