Including `-d:release` optimizes it, so that's probably what he meant. It's one of the first things in the [tutorial](http://nim-lang.org/0.11.0/tut1.html).

Writing "--opt:size" means to try to optimize for size, trumping optimizations for speed you get with plain "release" mode.

I'm guessing -d:release implies —opt:speed (on top of disabling various runtime checks)