Planning for efficiency early vs Premature optimization

Question

I seem to notice two schools of thought emerging in regards to optimization:

1. Premature optimization is the root of all evil. You should only optimize when you've written the most readable and simplest thing possible. If after profiling you determine that the software is too slow, you should optimize.
2. Optimizations should be done early in a project's lifecycle. Optimizations need to be planned for, but should be done reasonably.

On the face of things, they seem to be fairly opposed viewpoints. The thing is, I see the merit in both schools of thought. I can also think of times when both of these ways of thinking have helped me write better and faster software.

Is there any way to reconcile these two ideas? Is there a middle ground? Is there a time when one idea is the best tool for the job? Or am I presenting a false dichotomy and both views can co-exist peacefully?

Answer 1

Optimise at the design and architecture level early on. Micro-optimise at the implementation level later.

You need to be aware of the performance costs of design decisions you make which will be hard to change later. Implementation can often be tuned later on, which is why it's not worth doing it until you know that it's a problem.