Currently affects OS X and Linux.
GTK+ 2.24.23

I have around 2900 fonts installed.

I checked them out from the Google font directory and placed them in ~/.fonts:
https://code.google.com/p/googlefontdirectory/

I noticed that launch times of a certain program I develop for were then up considerably, mostly spending time in Pango and Fontconfig functions when I profiled them.

I managed to reproduce the issue in the attached file. It is a font lister that creates a combobox and styles fonts in the dropdown list.

Unmodified, with 2900 fonts, the program takes around nine seconds to start up fully (on my machine).
After commenting out either line 88 or 89 (doesn't matter which), the program starts immediately.

I dug through some of the GTK source code but I was unable to determine why it took so long when using both a separator function and a cell data function.
If you put a return statement at the beginning of the separator function no significant speed improvement is seen. If you put a return statement at the beginning of the cell data function the program launches almost instantly again.

That does not explain why using only one (regardless of which) would cause launch times to be practically instantaneous while using both would be around nine seconds slower.

The issue does not seem to be reproducible with GTK+ 3 <= 3.10, but earlier versions of GTK 3 were affected.

Steps to reproduce:
0.) Obtain several thousand fonts
1.) Compile fonts.c
2.) Launch executable -- observe startup time
3.) Comment line 89 in fonts.c, recompile
4.) Launch executable -- observe startup time
5.) Uncomment line 89 in fonts.c, comment line 88 in fonts.c, recompile
6.) Launch executable -- observe startup time

Expected results: performance runs for all three tests should be approximately equal
Actual results: performance run for first test takes an unexpectedly long time