The fastest effect is copy, for which no special calculations are necessary: the colour of each pixel is just copied directly from the castmember to the screen.
Matte, transparent, reverse, ghost, not transparent, not ghost and not reverse are of comparable speed, though the speed of matte may depend on the complexity of a sprite's shape, and it uses more memory than other effects except mask.
Not copy seems (oddly) to be significantly slower than the above, presumably because, despite the name, it acts as not matte. That is, instead of becoming a solid black, white space surrounding the image becomes transparent.
Mask is slower and uses more memory, because Director has to keep track of the masking region as well as the sprite image itself.
Blend can be excruciatingly slow, and the remaining effects (darkest, lightest, add, add pin, subtract and subtract pin) are similarly lethargic.
For a good feel for the different effects, examine the Ink_FX projector supplied with Director.