division... slow?

In a lot of videogame code, whenever a lot of numbers are to be divided by one number, I usually see this:

inv = 1.0 / divisor;
...and then all the numbers * by inv

rather than all the numbers being / by divisor.

I'm guessing, at the CPU level, division is slightly slower and less efficient than multiplication?  I can't really think of any other reason why someone would do this.

If divisor is a constant, and your compiler is not smart enough, this should increase performance. Division is more costly than multiplication. Depending on the processor design, the difference can be very large.