Argentina and Switzerland was the fourth of seven knockout games this year to go into extra time, and if the Netherlands hadn't pulled a late penalty from elbow-deep out of its ass, it would have been the fifth. This is way, way above historical trends.
Overall, 26.8 percent (55 of 205) of knockout matches in World Cup history have gone into extra time. This varies by round.
In the first knockout round 31.3 percent (25 of 80) matches have gone into extra time—though before this year, that number was 28.8 percent. In the quarterfinals it is 27.6 percent (16 of 58). In the semifinals it is 21.9 percent (7 of 32). In third place games, extra time is very rare—it's only happened 5.9 percent (1 of 17, 1986 being lone occurrence). In the finals it happens a third of the time, 33.3 percent (6 of 18).
So if you're one of those who's been bitching about all the extra time, history says you should be seeing less of it. That said, if it really is the Brazilian heat flattening everyone out, this year might just end up being one massive outlier.