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

A question on Lightroom. I read somewhere that Lightroom does not process again and again raw data after each user input. It demosaic once in the beginning and creates a 16 bit rgb bitmap with gamma 1.0 in the prophoto colorspace. Then every user manipulation is done on the bitmap which is already demosaiced. This is done because demosaicing is a time costly process. Am I right on this?

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Panagiotis