Why do pre-compiled headers make my build take longer?

Hi,
I just tried using a pre-compiled header for a large C++ build on Solaris to try to speed it up, but it ended up slowing it down (from ~94 mins to ~111 mins).

The method I used was to get a list of all 3rd party header files used in the project and stick them in 3rdParty.h which I pre-compiled and then I just changed the Makefiles to add -include path/3rdParty.h to all the g++ lines. This way I didn't have to change hundreds of .cpp files.

Is this the wrong way to use pre-compiled headers?
Why does this take longer?

Before speculating much, have you familiarized yourself with all of this information:

Precompiled Headers - Using the GNU Compiler Collection (GCC)

You probably have, but...

Yeah, I took a look at that page.

OK, I just added a -H flag to g++ and it's saying that the pre-compiled header isn't being used.

Code:

---

x /tmp/myapp/3rdParty.h.gch

---

It should have a ! if it was used.
I used exactly the same compiler flags to build the .gch file as I'm using to build the .cpp files (except the .cpp files also have some -I flags).

Crap! Nevermind. My .gch file was also using a -M option that the .cpp files weren't. Now it's using the .gch file.

OK, in case anyone is interested, I managed to cut the build time by 33% by only including the 20 top included header files (I used a script that counts how many times each header file is included) instead of including all 3rd party header files in my pre-compiled header.

So when using pre-compiled headers, choose which headers are pre-compiled carefully by measuring how many times each one is included.