rxvt-unicode extremely slow to start

[Richard Cobbe]

Some time within the last month or so, urxvt (from the rxvt-unicode
package) has become very slow to start, taking on the order of 20 seconds
to display the window after I launch it.  Once the window's up, everything
works just fine: the shell or whatever program starts immediately, the
window is responsive to user input, and the window disappears immediately
after exiting the shell or whatever.

I have no idea how to debug this.  Where should I look for information
about what's going on?  I don't see anything in Console or under
~/LibraryLogs/X11.  Nothing's printed to stdout when I start urxvt from the
shell prompt.  The only thing I've seen is that, according to htop, urxvt
sits at about 100% CPU usage from the time when I launch the process until
the window first appears.  Once the window becomes visible, the process's
CPU usage drops back down to nothing.

Suggestions about where else to look are most welcome.

rxvt-unicode @9.22_0, MacOS 10.11.6, XCode 7.3.1, XQuartz 2.7.9.

I experience this very consistently on three different Macs.

Thanks,

Richard