It shouldn't be dramatically more resource-intensive. There's probably a lot of room for improving performance, especially in Lil's garbage collector.
I can say the native version is much more likely to stick to rendering at the intended 60fps, whereas your browser may be dropping frames (which is mostly unnoticeable, most of the time).
The native version also currently uses SDL's software renderer. If you're on a Mac it's fairly easy to build from source, so perhaps try replacing SDL_RENDERER_SOFTWARE with SDL_RENDERER_ACCELERATED in src/decker.c? Only makes a negligible difference for the devices I've tested, though.