Thanks for your reply David.  My emails go through moderator approval, so they may appear out of sequence relative to yours.

> (That's still pretty slow, though. Loading 1200 small files shouldn't
> take 5 seconds. How large is your ~/.emacs.d/semanticdb directory?).

'du -sh' currently shows 51M for the semanticdb directory.  It may have increased in size in the last couple days.  I think I remember it being 41M yesterday.

The Boost headers account for 24788 KiB, determined from
   du -s `ls | grep boost` | sed 's/\t.*//g' | xargs python -c 'import sys; print(sum(map(int, sys.argv[1:])))'

A similar command indicates 17504 KiB for files within the project source tree.

> To verify, I created a test file including a whole bunch of Boost
> libraries. gcc says it has ~870 header dependencies, so that's not too
> shabby.

You are certainly getting better performance out of semanticdb-find-table-for-include-default.

Mine:  semanticdb-find-table-for-include-default                   1238        36.186072999  0.0292294612
Yours: semanticdb-find-table-for-include-default                   958         2.2813617300  0.0023813796

What was the value of your semanticdb-find-default-throttle for the run?