Most products today are made with bone (ossein) gelatin from cows. The isoelectric point is about 4.5 and this is the point of minimum swell. So, the gelatin is "tight" in acid fixers and diffusion in and out of chemistry and salts is restricted. At an alkaline pH, gelatin is "looser" due to higher swell and thus chemistry diffuses inward and outward more easily. Fixing is faster.

However, development can continue in an alkaline fix unless you either use a stop bath or a running water rinse.

Ammonia odor can be a problem for some users of TF-4 and thus it was reformulated to remove the solids in the bottle and to prevent (or at least reduce) the ammonia odor. Thus we have TF_%>