Law and its formula that says: more time organism working the slower (or less precise) the movements
Your question is a bit vague, but it sounds like you might be looking for Hick's Law.  Hick's law states the relationship between the number of possible responses that an organism can provide for a given task and the minimum time necessary to engage a response.

You may have also been thinking of one of the computational models for two-alternative forced-choice tasks (2AFC).

Let me know if any of this rings a bell!