that an acid stop DOES stop quickly.
I'm of the opinion that a water stop is as quick as an acid stop.
Two reasons: stop is 98% H2O. H2O compared with acetic acid
is a swift far traveling molecule. Dilution is quick. Second: H2O
is near neutral and so as it quickly enters the emulsion it
stops developer action.
The main reason for an acid stop is to maintain the acidity
of an acid fixer. Dan