What is shut up and play, already?
There are a number of reasons. It seems like there are a lot more 'Triple Stumper' clues(per the jboard.com site, that's where none of the three gets them right, or even rings in.) When you have several clues in one or two categories of dead silence, it adds up.
Also, Alex Trebek talks a lot more between clues than he used to, often making little comments/jokes about the clue('You STILL haven't remembered my FAVORITE animal, the musk ox!'), or reminding us where that contestant stands('And with that response, Tom, you move into a tie for second place with Dick, 200 dollars ahead of Harry. Select.')
I think the main thing that takes up time, even on shows when the contestants aren't stumped too often, is the rather long-winded explanations that crop up when the judges go back and review a response('Our judges looked it up and found that there were several people named 'John Smith', and since our clue wasn't specific enough, we're going to credit you with the response, but we won't penalize her, no harm no foul, yadda, yadda...')
And while CTListener's response comparing the contestants to baseball and football players was funny, quite a few contestants are in fact more 'deliberate' in their play, some making long pauses before they choose the next category(they'd probably be a PITA to be behind at a drive-thru fast food place), and, somewhat irritatingly, a lot of them slow things down by being overly precise and verbose in selecting categories. ("I'll take 'Famous 18th Century Monarchs' for twelve hundred dollars, please, Alex.", instead of a 'let's keep it moving' pace,shortened to 'Monarchs, 12'.) This strategy seems effective when contestants get clues right, but 'pausing dramatically' between not answering anything is a time-waster.