How does higher stellar metallicity affect the main sequence lifetime?

It increases opacity, speeding up fusion, thereby shortening the lifetime.

Metallicity, defined as the abundance of elements heavier than helium within the star, plays a subtle but significant role in determining a star's exact placement and evolution rate on the main sequence. Higher metallicity means the stellar material is denser and more opaque, acting like an insulator. This insulation traps thermal energy more effectively inside the star, leading to a higher internal temperature gradient, especially in the core region. Since the rate of nuclear fusion is highly sensitive to temperature, this core heating accelerates the rate at which hydrogen is consumed. Consequently, a star with higher metallicity burns through its core hydrogen fuel more rapidly, resulting in a shorter overall lifespan while residing on the main sequence compared to a star of identical mass but lower metallicity.