Law and its formula that says: more time organism working the slower (or less precise) the movements What is the name of the law and the equation that says that "the more time an organism is working the slower (or less precise) its movements." ?

I read once that there is a law in behavioral science that says that: "The more an organism is doing something(working) the slower its movements are (or less precise, can't remember exactly).". That law also has equation that describes it. I need to know the name of that law.
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!