A PR is indeed likely the right way forward! There's even a ongoing Racket summer event focused on examples as well. 
In case you may not have encountered it yet, Racket docs will reveal their source path if you click on the section header. So for the page mentioned above, you can see that the main document is https://github.com/racket/racket/tree/master/pkgs/racket-doc/scribblings/guide/guide.scrbl, and then from there, a little searching / guessing reveals the part in question: https://github.com/racket/racket/blob/e9376c099a53825ef31f233ca80f42641ebaa26a/pkgs/racket-doc/scribblings/guide/performance.scrbl#L365