The U.S. political system, with its high number of veto points, is arguably unsuited to taking decisive action in response to a crisis compared to alternative models, such as the Westminster system in play in the United Kingdom and Canada or to the multiparty coalition systems of northern Europe. It’s hard to know how to evaluate that claim. There is, however, a political science literature indicating that American-style systems are more prone to total constitutional breakdown in a crisis.
I can't comment on the political science literature, but it seems to me that the U.S. doesn't do any worse than other developed countries on this score. You can argue about whether our historical responses to immediate crises have been correct, but they certainly seem to have been as decisive as anyone else. To pick the example of our current economic meltdown, which countries have done better? Japan? Germany? China? Iceland?
There's a pretty good case to be made that these countries have all acted both more slowly and with less sense of genuine urgency than the U.S. At the very least, it's safe to say that almost no one has done demonstrably better. We do indeed have a large number of veto points in our political system, but in practice it's not clear that this has prevented decisive action during a genuine emergency.