[gabemart]

My personal approach is to do some order-of-magnitude performance testing as early as possible, to validate that the approach chosen to solve a particular problem is at least tenable.

If you ignore performance completely until late in the project, you can paint yourself into a corner. This includes cases like knowing that performance is 50x slower than will be acceptable throughout development, but saying "we can add X later for an easy performance win". If you don't actually test that X gets you within reach of your performance target at an early stage, you can end up with a fully built system that is unusable.