What Rhysati said. As you level up you get gear that lowers your GCD (especially Pugilist/Monk as you get more Greased Lightning stacks) as well as more skills that are off-GCD so you have more that you need to do and keep track of.

MNK is probably one of the hardest jobs to master because you need to be quick and keep a lot of buffs and debuffs up to maximize your damage.