The paper deals with a problem of control of a system characterized by the fact that the influence of controls on the dynamics of certain functions of state variables (called observables) is relatively weak, and the rates of change of these observables are much slower than the rates of change of the state variables themselves. The contributions of the paper are twofold. First, the averaged system whose solutions approximate the trajectories of the slow observables is introduced, and it is shown that the optimal value of the problem of optimal control with time discounting criterion considered on the solutions of the system with slow observables (this problem is referred to as perturbed) converges to the optimal value of the corresponding problem of optimal control of the averaged system. Second, a method for constructing an asymptotically optimal control of the perturbed problem on the basis of an optimal solution of the averaged problem is indicated, sufficient and necessary optimality conditions for the averaged problem are stated, and a way for constructing numerically a near optimal solution of the latter is outlined (the construction being illustrated with an example).