Different types of alcohol contain differing proportions. 4% alcohol will get you drunk faster than 2%, partly because the equivalent amount of alcohol in each sample will mean that the 2% sample is more liquid (water, possibly flavorings) than the 4% sample. Your digestive system is working on all this stuff, and it takes energy from your body to do its work. The more stuff you have to digest, the more (and longer) that takes.
I believe you get a faster buzz with caffeine-laced drinks, too (Irish coffee, anyone?), although I'm not certain of this.