A big subplot to the Texans last season was a perceived inconsistent offense, one that could only truly function in the second half. While the Texans certainly didn't make the most of every possible drive, the fact is simply that their defense was so atrocious that even a minimal amount of improvement there would have been enough to win two or three more games. However, there are oppositionalists and naysayers who would like you to believe that the Texans offense didn't milk enough time off the clock last season, and that when they went on a streak of three-and-outs, the defense suffered for it.
↵Ignoring the obvious bias involved there (i.e. the part where the defense was so bad that letting it get on the field more times was going to greatly increase the chances of them giving up points), I think this is more of an anecdotal point than a true reason. Football players get tired, yes, but keep in mind that these are the most well-conditioned non-Albert Haynesworth athletes on the planet. I'd argue that while it makes sense that a normal human would tire easily under these conditions, an NFL player will not necessarily follow that trend.
↵Football Outsiders ran a guest study yesterday that went over the correlation between a team's defensive DVOA and the amount of plays the offense spent on the field per game. Conclusion? Statistically insignificant. While the study is far from conclusive and definitely needs more controlled factors (time elapsed, for one thing) to be considered a significant breakthrough, it definitely makes for an interesting read.
↵I would tend to side with the study, but as it already fits with my preconceived notions, I'm sure I am biased to an extent.