RAF files contain (or at least can be configured to contain) a full size JPG embedded in them that will have the film sims baked in (as all JPGs do). You can use EXIFTools to extract them. Immich can be configured to make thumbnails off these embedded images instead of the demosaic'd RAF. I'm not sure if they are progressive JPGs but if they are I imagine it would be pretty efficient to directly read the images at a lower res and not even have to generate thumbnails (although that probably is less generalizable as a pipeline).

This is my current understanding and may be inaccurate.