Constitutional restrictions typically hinder efficient government operations by making it more difficult to get laws passed. They tend to do this in order to prevent the government from acting too easily and, thus, being more able to tyrannize the people. The United States' system of separation of powers and checks and balances is a good example of this.
In the US system, a law has to be passed by two houses of Congress (which can, as they are now, be controlled by different parties) and then be signed into law by the President. The President can of course be of a different party from those in control of one or both of the houses of Congress. This makes it very difficult for anything to get passed. We can see this right now as the Democrats (President and the majority in the Senate) and the Republicans (controlling the House) cannot agree on how to cut the budget deficit.
This is very inefficient. If we had a system where one party had complete control at any one time, we would have some sort of solution to the problem of the deficit by now. However, this was seen by the Framers of the Constitution as a good thing because it means that there has to be a fairly broad consensus in the country before things can get done. If this were not the case, a smaller group of people could force through policies that would be seen by some as horrible and possibly tyrannical.