[CJefferson]

GCC has had precompiled headers for years. They don't really help in C, bceause the time spent parsing headers isn't a significant chunk of runtime in practice. They can be useful in C++, as it's common to include massive amounts of templated code in headers, but they still only buy you 20% at best.

[eek04_]

For speed-architected C compilers/linkers lexing time is typically a very significant part of compilation time. The Unix compilers typically hasn't been speed-architected, instead having a focus on simplicity and ease of porting.