Patrick, your War Department Technical Manual isn't kind enough to say why. I think the most likely explanation why p-aminophenol's rate of development declines is because its oxidation product is acidic, and theby requiring external source of electrons.(posted 8160 days ago)The low restraining effect of KBr would apply to superadditive combinations with hydroquinone and some other reductants. It really depends on the affinity of the developing agent at the working pH to silver halides, water, and KBr. Bromide ion likes to adsorb on silver halide particles. This slows down working of HQ, ascorbate, etc. but doesn't bother phenidone. HQ at pH of 9 is mostly undissociated and won't reduce silver halides directly to any appreciable degree. Ascorbate almost entirely dissociate one -OH and is active, but for some reason will not develop silver quickly. However, ascorbate reduces things dissolved in water effectively. The reason I think is because ascorbate has high affinity to water and doesn't want to bump into silver halides, and even if it does, it won't adsorb onto it. This is very different from phenidone.
However, in print developers, condition can be different. Silver halide compositions and its physical structure can be different; sulfite is usually moderate; higher pH; etc. Plus, you usually add more bromide to paper developers. Some visible restraining action is not too surprising.