An enzyme is protein and it needs to maintain its 3D structure as the enzyme work like "lock and key". A little deformation to the enzyme structure can disable its functions.
Protein is prone to denaturation. Condition like pH and temperature can affect the protein and cause an alteration to its structure. Higher temperature should speed up the reaction but too high temperature might damage the enzyme. So, the "optimal" condition would be the condition where the enzyme structure is not altered.