X11 font cache on OS X Often when I open an X11 application (Wireshark, GIMP) it has to build some sort of font cache, which often takes 30 seconds or more during application launch. I don't change my fonts often and Wireshark definitely doesn't need access to hundreds of fonts. 

Can someone explain what this font caching is, why it impedes the launching of applications (instead of being some background X11 process), and why it needs to happen so often?

I'm not sure whether this is specific to OS X.
Font caching is an operation done by the font rendering library, called by the application program.
In theory, it would allow for faster rendering, since list of available fonts on the system is already created, the system could even temporary save commonly used fonts for faster access.
On OS X, the position of directory for font cache within the library can be misconfigured, so the application program can not write the font cache. So font cache is created on the fly, every time you run the program. This is slow.
You can solve this problem by running your application as administrator (root) by typing "su gimp" once. Do not work in administrator mode, just run it once. In this way, font cache will be created (as administrator has root permission on font cache position), and the next time you start the program (as normal user), it will be faster.