Yeah, working up to white is really slow. If you want a bit of speed and still really good results flip it and start with pure white, then add the darker colours through several controlled washes of gradually greyer tones, taking as much care as you can to get them in just the very recessed areas. Finally touch up any minor errors with more white. A hint of blue in the mix can give the white a more lustrous look, and also allow your lightest shading to come more into the white without making it look too grey.