If you start with 'reactants' ie LHS only, initially forward fast, reverse slow. Eventually forward and reverse rates will be equal (dynamic equilibrium)
We will assume that at the start of the reaction, we only have 'reactants' ie LHS. Therefore the only reaction possible is forward. Reactants are also in the highest concentration so the forward rate will be at its fastest.
As the reaction proceeds, more 'product' will be formed, and therefore the reverse reaction rate will increase, i.e. forward rate starts at its highest and decreases, reverse rate starts instantaneously at 0 and increases. This is simply a function of the probability of collisions between reactant or product particles changing as the relative concentrations of these shift.
Eventually the rate of the forward and reverse reactions will be the same and the concentrations of all substances will reach constant values, even though reactants will continually form products and vice versa. We have now reached a state of dynamic equilibrium .