A couple questions:

First: I assume that reading images from [pix_buffer] is less CPU
intensive than decoding a video file with [pix_film]. Is that true?

Yes. Reading and writing a pix_buffer is at most a memory copy operation.

Second (and assuming the first statement is true): is there a way to
dump a whole video into [pix_buffer] without doing it frame-by-frame?

On OSX you can load a compressed video file into RAM using the 'ram' message. This will use less memory than pix_buffer but still use CPU for decoding the video.