Hours? Either they have several gigabytes of CSS output (ouch) or they're using a ZX-Spectrum as a build machine. Or, possibly, they're constantly recompiling,

Hours? Either they have several gigabytes of CSS output (ouch) or they're using a ZX-Spectrum as a build machine. Or, possibly, they're constantly recompiling, like doing a change in SASS a hundred times per second. Otherwise, interpreted script-based parser (given that it's written properly, algorithm-wise) should be sufficiently performant to produce results in a sane time.