There is an incremental person who, when added to a project, makes it take longer, not less time.
Brook's law, by Fred Brooks, Mythical Man-Month

One key to effective programming is learning to make mistakes quickly, learning from them each time. Making a mistake is no sin. Failing to learn from a mistake is.

Steve McConnell, Code Complete, Second Edition

It's easy to confuse motion with progress, busy-ness with being productive. The most important work in effective programming is thinking, and people tend not to look busy when they're thinking. If I worked with a programmer who looked busy all the time, I'd assume that he was not a good programmer because he wasn't using his most valuable tool, his brain.

Steve McConnell, Code Complete, Second Edition