When I click the font chooser button in any GTK3 application, it takes around 10 seconds to open the selector window.

I demonstrate it with gtk3-widget-factory on this video:
https://www.youtube.com/watch?v=RWhuNwPAX6I

I attached strace to gtk3-widget-factory and the process generates sendmsg calls during the "freeze". 

Here is a syscall count output:

(~) % sudo strace -c -p 9404               
Process 9404 attached
^CProcess 9404 detached
% time     seconds  usecs/call     calls    errors syscall
------ ----------- ----------- --------- --------- ----------------
  0.00    0.000000           0        38        15 read
  0.00    0.000000           0        43           write
  0.00    0.000000           0        15           open
  0.00    0.000000           0        15           close
  0.00    0.000000           0        36        22 stat
  0.00    0.000000           0        27           fstat
  0.00    0.000000           0      2771           poll
  0.00    0.000000           0        29           mmap
  0.00    0.000000           0        21           munmap
  0.00    0.000000           0       215           brk
  0.00    0.000000           0       789           writev
  0.00    0.000000           0        13           access
  0.00    0.000000           0         3           recvfrom
  0.00    0.000000           0     40658           sendmsg
  0.00    0.000000           0      5345      3964 recvmsg
  0.00    0.000000           0         1           uname
  0.00    0.000000           0        24           fcntl
  0.00    0.000000           0         1           readlink
  0.00    0.000000           0        15         2 futex
  0.00    0.000000           0         1           restart_syscall
------ ----------- ----------- --------- --------- ----------------
100.00    0.000000                 50060      4003 total


I have quite a few fonts installed, most notably from this repository:
https://github.com/w0ng/googlefontdirectory/tree/master/fonts