RAW imagery I have worked with is about 25-50% losslessly compressible on average. Most of your gains in image compression are from quantizing the gamut in clever, (usually) imperceptible ways.

Raw imagery contains a lot of entropy that usually doesn't affect the appearance perceptually, but still has sig figs that frustrate compression.

My Fuji has lossless RAW compression and as far as I know quite a few others too.

I just tried compressing one with 7-zip ultra level compression. Saved maybe 5%. Wouldn't get even that with realtime compression.