I know that it is not desirable for behavioural data to show a (within-subjects) speed-accuracy trade-off, as it can lead to e.g. the wrong interpretation of a single subject's decreased RTs as meaning improved performance, when in fact if one looks at error rates, one can see the increased speed came at the cost of increased error rates.
However, what I don't understand is why, assuming that subjects are instructed to respond slowly enough so that their error rates approach zero (so not the usual speeded responses paradigm), why would it not be correct to then analyse the group RTs, which will probably show wide between-subjects variations and can thus offer meaningful interpretations? This way, it would not matter that all subjects will have traded speed for accuracy, as it is precisely the amount of "slowing" that becomes the dependent variable.
Hope the question makes sense, thanks!