The speed of a chemical reaction is affected by temperature, concentration, particle size and the presence of a catalyst. It can be calculated by measuring changes in reactants/products.
In chemistry, graphs can be used to follow the course of a reaction. A graph can tell us many things about a reaction.
The graph below shows two similar reactions.
The magenta line has a steeper gradient and represents conditions favouring a faster reaction than the red line. When the reaction is finished (the end-point represented by the dotted line) the graph goes flat as no more products are being produced.