Well the first answer is giving you a statistical answer, I'd like to give you the reasoning.
The world’s laziest Pokémon. When it is lounging, it is actually saving energy for striking back
This is the pokedex entry from DPP and B&W.
Basically, slaking is charging up when it is not moving, allowing it to strike back immediately, the moment the turn of "loafing around" is over. This is why it was given a base speed of 100.
Of course, this doesn't explain how using skill swap or some other method to remove truant from Slaking still allows it to be so fast, because logically, this would slow it down as it would not get a chance to charge.
I guess that statistics and logic cant always match up, but there's decent reasoning behind making it a fast Pokemon I suppose.