Efficient means with a minimum of excess effort, cost, time involvement, etc. If all governmental power were centralized in a single branch, there would be reduced need for coordination of communication and responsibility between different divisions of the government. There quite possibly would be fewer bureaucrats involved in carrying out governmental functions. The cost of paying for support of the government would almost certainly be reduced. So, for these reasons, yes - the government might be more efficient.
Effective means adequate and able to accomplish the goals of an organization or individual. One of the primary goals of the Founding Fathers as they were developing the governmental structure spelled out in the Constitution was the prevention of all power becoming concentrated in one place. Based on that goal alone, the idea of having a single branch of government is completely ineffective at meeting the wishes of the original citizens of the USA.
Instead of being more efficient the government will become less efficient as only one branch would have to do all the other 2 branches work example if the legislative branch was handling the executive snd legislative it would fail as it would not provide jobs for others maybe and it would be less effective