A classic way to calculate normals as Poser4 does, there are other ways to do it with different visual results.
The code is in C ( I use Begin for { and End for }).
I don't see any problem with speed for this, the only part that is worth to optimize is NormalizeVector that requires division and square root that is slow. I can be done in Assembler with SSE and it will be very fast..

Stupidity also evolves!