Ok here’s some further info/possible repro case if it is of use:
I built my own emacs by doing a bzr branch bzr://bzr.sv.gnu.org/emacs/trunk,
then autogen.sh and ./configure —with-ns
I removed my .emacs file so as to get default settings, then launched emacs,
opened a large text file, and subdivided the frame into several windows.
With this setup it is quite easy for me to get the slowdown to happen by just
dragging a divider around for a bit.
Here’s a clip of a slowdown with the activity monitor visible:
https://www.youtube.com/watch?v=olkyqVOWSLs
You can see that emacs is only using a few percent cpu throughout the slow redraw,
whatever that may imply.
I’ve also attached a sample I took of emacs during such a slowdown.
It looks like a lot of calls are blocking in _CGSSynchronizeWindowBackingStore
under the hood.
Hope this is helpful.  Please let me know if I can provide more info.
Thanks
-Eric