Q. How do I speed compile time for large application such as open office or kernel?
A. First, you cannot just increase the speed. Sure there are some hacks out. In order to work following instruction you need to have a powerful system or SMP system with lods of RAM.
You can improve performance by modifying Makefile for parallel build. Use GNU make utility to maintain groups of programs. You need to pass -j option to make command. It specifies the number of jobs (commands) to run simultaneously. If there is more than one -j option, the last one is effective. If the -j option is given without an argument, make will not limit the number of jobs that can run simultaneously.
$ make -j5
Please note that lots of software are not designed to build as a parallel build. Read man page of make for more information.