By being anti-aliased it means they have to "alpha-blend" the character bitmap with the background and foreground color, which is several times slower than the simple case they had to do before. I don't believe this was used on 8-bit color displays, as it would add a "dithering" step.
Scalable fonts just add a step in the generation of the characters, which can then be put in cache and only hurt performance the first time they are generated. And some scalable formats included pre-generated bitmaps for the common sizes. Windows would happily return a font of this already generated sizes, instead of generating a new one, if the size we wanted was close enough.
Anyway, just to point out one of the many things we now take for granted that are possible because we have the CPU power, and the memory, for it.