Getting rid of defensive copying that ended up in a hot loop allocating a few gigabytes per second of temporary copies sped things right up. That stands out in

Getting rid of defensive copying that ended up in a hot loop allocating a few gigabytes per second of temporary copies sped things right up. That stands out in my mind, but I know I've also fixed things that were accidentally n-squared and fell over pretty quickly in production. The other thing was actually used for some time though.