McMaster-Carr begs to differ. Hell even old.reddit is pretty snappy (but deliberately shittily rendered on mobile). Websites can be fast if you don't stuff them with bullshit or degrade then on purpose to drive traffic to the app.
But if they had a native app (do they?) I imagine they would have the wherewithal to build the app natively, with the same stellar navigation of their website, and maybe some native-only features? Imagine if you could use the 3d sensor + camera of an iPhone, and point it at an assembly, and the app would identify the parts it could, and you could order with one click, or integrate with a local ERP or other systems...