Many projections require the calculation of sine and cosine of some angle.
The evaluations of sine and cosine are very similar and there is significant overlap in the series. Multiply this by a few million calls and we are talking real speedup gains. For example, the Cassini proejction on a Sun runs 2.5-3 times faster than GMT3.2. This is also due to some other efforts to gain speed. We know under Solaris, users can use the -lsunmath library which has sincos.