Much of the speedup is gained by manually eliminating (only possible if the entire function can be reduced to 5 bytes or less), or at least simplifying calls along the critical code paths as far as possible. This doesn't even produce nearly as good results as an optimizing compiler could have because of many restrictions a compiler doesn't have to deal with in the first place, so every optimizing compiler can do and usually does an excellent job at this if told to do it. Skyrim would probably experience an execution speed gain of over 100% just by applying this single optimization, as it has drastic consequences to the amount of code that could be detected as being redundant and thus completely eliminated. I know that sounds exaggerated, and normally would be, but it isn't when you've read and profiled enough of the code to know just how bad the compiled code is.
