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We're all warned against premature code optimisation, and that's fair. Optimising too early can make the code more complex and hard to maintain without any evidence as to whether it actually makes it faster. A lot of the time it really is better just to write the code and worry about optimisation when/if speed becomes an issue.
On the other hand all programmers ought to understand the concepts behind Big-O notation and algorithmic complexity. Applying these concepts to your code can actually help make it both more simple and faster at the same time. It may mean not going with the first solution you think of, but once you've got the concepts worked out, making these simple changes to your coding style will allow you to reap huge benefits.