Slowdown (hang) with large c++ files

[gga]

I am trying to fix an issue with GNU emacs that is driving me insane
which is a severe slow-down (a hang of 10 seconds or more)  whenever
I've loaded a large C++ buffer (100,000+ lines or more -- a file
autogenerated by swig).
The hang happens sporadically for no reason (no user action triggers
it) and happens even if the buffer is not being displayed.
It may be font-lock issue, but I am unsure how to limit font-lock to
parse only a couple of lines.

Can anyone help me out or suggest stuff to try?