I believe what he means is that when he uses tools to recompile the games using the new version of GCC (or XCode, or whatever he is using), it takes 15 or 8 seconds depending on if optimizations are on or off. If optimizations are on, the compiler has to more closely exmaine the code, and try to order it in such a way as to run the fastest on a system which it is being compiled for. With optimizations off, the compiler is in dumb mode, where it just compiles the code for a base instruction set architecture.
Having optimizations turned on should make the compiler take longer, as it is in a more complex mode. Having such a fast compile for their games is simply incredible.
You can optimize dozzens of things for different hardware setups. Some computer have more registers, which are the fastest memory. Optimizing to use more registers means less RAM access, which will speed up the program. Other things like filling Branch and Load Delay slots with instructions that can go there (instead of nop, no operation, instructions which just waste processor cycles), can significantly speed up a program. Moreover, activing things like instruction prefetch, organizing the code, and more can help speed up a program.
These things can be done by the compiler with RISC computers, making the life of a programmer much easier, but the life of a compiler (and those who write them), much more difficult.
I'm sure this is more than you wanted to know, but at least you know it now
