"You can't tell where a program is going to spend its time. Bottlenecks occur in surprising places, so don't try to second guess and put in a speed hack until you've proven that's where the bottleneck is."
Moreso, in my personal experience, I've seen a few speed hacks cause incorrect behavior on more than one occasion.
Parent is talking about building software that is inherently non-performant due to abstractions or architecture with the wrong assumption that it can be optimized later if needed.
The analogy is trying to convert a garbage truck into a race car. A race car is built as a race car. You don't start building a garbage truck and then optimize it on the race course. There are obvious principles and understanding that first go into the building of a race car, assuming one is needed, and the optimization happens from that basis in testing on and off the track.