Why would the framers of the U.S. Constitution want to limit the powers of each branch of government?
The framers of the Constitution understood from an experienced point of view the problem of allowing any one man or group to have unyielding power. To ensure such tyranny did not become part of the new government they instituted numerous checks on the power of the government. The limitations placed upon each was twofold. It required the partitions of the federal government to work in concert according to the mandates of the people and it ensured no portion of the government had the power to trample individual liberty.
The three branches of the government have different and if unchecked vastly powerful divisions of the government. The Executive Branch, consisting of the governing officials such as the President are controlled by the people through the vote as well as the other two branches. This limits the ability of the President to make decrees outside the bounds of the Constitution. This was a very real concern of the founders having experienced the oppression of the British Crown.
The Legislative Branch, made up of Congress has the power of the purse. By controlling the federal budget and enacting legislation the group could quickly overwhelm the public with burdensome mandates. They are controlled via the vote, and the other two branches to control their power.
Finally the Judicial Branch, the courts have power to send innocent people to jail. This was perhaps the second greatest concern of the founders. Some courts are held in check by direct vote, but even those appointed are subject to disciplinary measures if they are found to act outside their jurisdiction. Another key aspect in controlling courts is the Executive ability to grant pardons.
The balancing act the federal government is forced to endure certainly slows down the process and deliberately so. The government was designed to be slow to act to ensure no one's rights were violated. Ultimately the states as a collective have a check over the federal system as well through the ability to amend the Constitution when needed. The founders knew that two heads were better than one and a hundred were better than two. They compromised on speed, but took security in the thought that a slow right was better than a quick wrong.