Super Fix I fixes more rapidly than any other fixer, but wash times are just slightly reduced over other formulas, say about 20%. Super Fix VII fixes at about the same rate or just a tad faster than Super Fix I but washes out twice as fast thereby reducing water consumption by 50%.
These are rough approximation comparing apples to oranges, so the real fixers are quite different and behave differently. Synergy between fixing agents is used to gain fixing speed in Super Fix I, but a swelling agent is used in Super Fix VII in addition to another type of synergy to speed fix rates and wash rates. With Super Fix VII, we are approaching the limits of both fixation and washing due to the limit imposed by the diffusion rates of the chemicals into and out of film and paper emulsions.
PE