What causes caffeine to take so long to take effect on me?

Caffeine metabolism: Caffeine is metabolized by a liver enzyme. If you have a particular common gene mutation what is known as a snp (single nucleotide) polymorphism) you may be a rapid metabolizer of caffeine, which means it gets broken down more quickly generally requiring you to have to take in more before feeling any effect. If you were a slow metabolizer you would feel the effect more quickly and stronger.