The more the solution of a reactant is diluted, the slower the reaction will occur.
The key thing you need to understand here is that chemical reactions depend on reactant particles bumping into each other (collision theory).
The more concentrated a reactant is, the more likely it will be to bump into other reactants and product chemical change.
Here is video of a quick lab performed to test this. In the video, varying concentrations of #Na_2S_2O_3# are allowed to react with HCl. A timer is included in the video so you can see how the changing concentration changes the overall speed of the reaction!