I also sometimes get irritated about a slow pace of play, but as someone said, the clock is there for a reason, and there are at least two situations, where playing slow is kind of a legitimate strategy.
For first is in multi table tournaments, when the bubble or an important pay-jump approach. Small and middling stacks on a slow table has a better chance to survive, so it makes sense for them to slow down action. This is why, hand-for-hand play exist, but there are also clear advantages in slowing play down before the actual bubble or payjump. Sure I understand, this can be annoying, if you just play for fun. But its not against the rules, so you kind of have to accept it.
The other situation is to not give away timing
tells. If you check very fast, your opponents know, you were not even considering a bet, and this does tell them something about your hand. And the same if you call very fast. Then they know, you were not even considering either folding or raising. So before checking or calling, you should typically do at least a little bit of "Hollywooding" to pretend, you have a decision. Maybe not 30 seconds, but you should not insta click either, unless you are folding.
players that take too long for a simple check, EVERY single play they take almost 30 seconds to act! 
