this certainly sounds like something in your setup is broken, and i’d recommend you follow the previous advice of checking with -d perf what is going on.
“only base curve active” is misleading, to go from raw to picture you’ll certainly need more modules than that (which you may not be aware of).
that said, to answer your questions: yes, dt is processing only the visible pixels only at the current scale (zoomed in: just a portion, zoomed out: just low res). pipeline stages are cached, only the active module onwards is processed.
if you’re truly into speed, what would need to be done? rewrite the whole pipeline for the GPU. as it turns out, image operations work very closely like the hardware is built, the texture units are a big help, and just avoiding the occasional cpu/gpu buffer copies and the bulky software stack cairo->gtk->x11->screen are a major performance boost over current dt’s opencl implementation.