What is the meaning of "If only ... were this efficient"?

Solution 1:

It refers to something that has been done efficiently (in the writer’s opinion) and means “I wish strongly that other people, whom I think are inefficient, were similarly efficient in what they do”.

Note the use of “If” and the subjunctive “were”, together expressing desires, possibilities and the abstract rather than present reality.

The use of “if only” is subtle. “If only stone were cheese, we could eat it” means that stone is not cheese. In your example, the construct means that the writer feels other people are not presently efficient.

Solution 2:

I cannot understand the meaning of "If only other people were this efficient."

"If only other people were this efficient." is half of a conditional sentence and the reader supplies the other clause. (Technically, the example clause should be preceded or followed by an ellipsis "...".)

"If only other people were this efficient, then the world would be a better place/ then everyone would be happy/ etc."

Or

"The world would be a better place / everyone would be happy / etc, if only other people were this efficient."

As an example:

A: "I can't find my car keys..." B: "If you look on your desk..." = "You will find them if you look on your desk", or "If you look on your desk, you will find them."