Thank you for your answers so far!
Sorry, I said that imprecisely. I meant that the base curve is the only user-selectable module I enabled (white in the pipeline). I understand that demosaicing, color space conversions, etc. are always active (grey in the pipeline).
I’ll check the performance profiler when I get back home tonight. Maybe something is broken indeed–although I did test on multiple operating systems with different GPU/CPU setups.