Reaction kinetics: explain question

A student investigates the reaction between aqueous sodium thiosulfate and dilute hydrochloric acid.

$$\ce{Na2S2O3(aq) + 2HCl(aq) -> 2NaCl(aq) + SO2(g) + S(s) + H2O(l)}$$

The student uses a cross drawn on paper under the reaction flask and records the time taken for the cross to become completely obscured by the sulfur precipitate. Two experiments are carried out under identical conditions except that the concentration of sodium thiosulfate is different.

Using the collision theory, explain why increasing the concentration of sodium thiosulfate decreases the time taken for the cross to be obscured.