much slower "make web" processing

[Graham Percival]

OSX, GUB 2.11.43, current git, from a "make distclean; make web;
make web"

Building the docs now takes a *lot* longer.  lilypond-book takes
about 30 seconds to look at each .itely file when that file has no
changes.  Previously, the "REading foo.itely... dissecting...
Writing snippets..." messages for /all/ the docs flashed by my
screen in about 5 seconds.  (assuming that there were no changes)

Python is using about 5% of my CPU, and my disk activity is about
1.5 MB/s, so I'm puzzled as to why it takes so long.  This isn't
an urgent thing -- I can leave it processing in the background for
five hours; it doesn't use up much system resources -- but it's
definitely a step backwards.

Cheers,
- Graham