The "inefficiencies" in our government are expressly written in to the Constitution. They probably feared a runaway Legislature as much as a too-powerful Executive (it's clear they regarded the Judiciary as "the least dangerous branch") and so they set up a system of checks and balances among the 3 branches which continue in effect to this day. President appoints, but Senate must confirm / 2 separate Houses of the Legislature, elected separately and for different term lengths / President is Commander in Chief, but money is authorized by the Legislature, etc. The add-on of the Bill of Rights further limited federal power ("Congress shall make no law....") and reserved powers to the states and to the people.
Finally, if slavery was America's "original sin," the Constitutional process of Amendment provided the means to expand rights to all Americans. Is the Constitution grounded in "liberalism" or "conservatism"? The good news is that any and all American"isms" are reflected - a strong commitment to the rights of the people as well as a built-in bulwark against the popular will run wild.
It may surprise many Americans to learn that our young country boasts the oldest written Constitution of any country on the planet! The Brits have a line of older documents starting with Magna Carta, but no one document which does it all. Other countries have copied parts of our Constitution, recognizing how well it has served us. I was sorry to read not too long ago that one of our Supreme Court Justices was understood to be disparaging in her remarks about the US Constitution. I suppose there are some documents which may seem to afford better protection for their citizens, but words are cheap: our Constitution is a model of careful construction, balance and idealism and will continue to serve us if we remain true to our uniquely American heritage.