Combustion is a chemical reaction between substances, usually accompanied by the generation of light and heat in the form of flame, The rate or speed at which the reactants combine is much high, because of the nature of the chemical reaction and also more energy is generated then can escape into the surrounding medium with the result that the temperature of the reactants is raised to accelerate the reaction even more.
Respiration is a series of chemical steps and is slower than combustion which is a single step.