I believe diffusion have an important role in photographic processes.
Fixer is a total reaction and temperature and time are not critical.
Fixer time is made during the clarification of the film plus
time required for chemical products formed to diffusion the film in fixer ( x 2 time of clarification).
It is best to follow the prescribed temperature.
From a practical standpoint, plus or minuc few degrees C for fixer is not a catastrophe.
A better agitation of solution fixer can compensate for the absence of several degrees.

George