Thread Coating: Modern spark plugs often come with a special plating or coating on the threads designed to provide corrosion resistance and ensure proper torque. Applying anti-seize compound can interfere with this coating and alter the torque values, potentially leading to over-tightening or under-tightening of the spark plugs.
Heat Dissipation: Anti-seize compounds are typically greasy or oily substances, which can act as insulators. When spark plugs are installed with anti-seize, it may interfere with the heat dissipation from the spark plug threads to the cylinder head. This can potentially lead to overheating of the spark plug and affect its performance.