Brakes work on the crude principle of friction between two surfaces. To give a uniform measure of the frictional characteristics of a Brake Pad the SAE J866A test procedure is used to give a two character code e.g EE FF GG HH.
Obviously Pads are only as good as the condition of the whole braking system. When first used, brake pads can sometimes give poor braking, as residual moisture from the manufacturing process works its way out and the pads bed in. It is best to bed in pads with gentle use and very heavy initial use can result in glazing, a condition where the resins in the pad crystalize with resulting poor performance and squeal.