Republican U.S. presidential candidate businessman Donald Trump speaks during the Republican presidential debate in Las Vegas, Nevada Dec. 15. (REUTERS/Mike Blake)

One of the most important factors for Iowa Republicans as they decide on a candidate is debate performance, according to Des Moines Register/Bloomberg polling released on Wednesday. Eighty-four percent of respondents said debate performance was important, with more than a third calling it extremely important. That's lower than the percentages looking at how the candidates reacted to events in the news, but is higher than the percentages who find media interviews -- and even speeches -- important.

This is what voters say, anyway. Voters, like anyone, like to say that they are making their decisions based on things beyond how entertaining or interesting or likable a candidate is, despite that often being the actual motivation. For the most part, candidates' poll numbers haven't moved a whole lot after a debate; it's mostly over the long term that the numbers have gone up or down.

We pulled national polling data from Real Clear Politics for the week before and the three weeks after each of the debates on the Republican side so far. You can see how the shifts after the debate are subtle -- and often slow to appear, thanks to the delay of putting a poll in the field after a debate and then making it public.

Last week, we pointed out an exception to this idea. When Ted Cruz ripped the media during the CNBC debate, it (coupled with Ben Carson's implosion shortly after) appears to have been a moment to which viewers responded.