At Moovweb, some of our customer's projects can take 30 seconds just for the Sass to compile for fairly straightforward sites.
Even if they recompile everything... all CSS needs to be interpreted by the browser at some point. How much CSS do they have?
That being said, what are the speed comparisons like?
Though I completely get the portability aspect.
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.