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

A couple of things come to mind. Firstly, your laptop is very slow. 10 seconds is glacial. I am not a Mac user but I suggest you run whatever performance monitor you have available in the OS while doing those tasks and see where the bottlenecks are. By comparison just now I exported a 800e raw and wrote it to a JPEG file on a USB stick in around a second. Rendering a D800e raw file in Library mode is pretty well instant and then the re-render in Develop mode is less than a second. I also work with big (2-3 Gb) tiffs and even they do not take much more than 1-2 seconds to re-render in Develop module.

Secondly, the camera is not rendering raw files. It is showing you a jpeg that is embedded in the raw file. This jpeg is also  shown to you in the Import module of LR where to get to choose which files you want to import off your card. You can click on this thumbnail in the Import Module to see the 100% view of it. You will see how big it is (quite decent size from memory) and it will (at least should) "render" instantly on event the most puny of computers, just like in-camera.

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Alistair Owens
http://www.alistairowensphotography.com