> At first, I thought it might be due to epoll/kqueue **TIP458**

While I still don't understand what "it" refers to, I did look up
TIP #458 and I agree that whether epoll is used in the notifier
is totally unrelated to the time required for updating graphics.
The graphics updates do not involve the notifier at all. No events
are being processed.  No files are being read or written.  And,
even if the notifier were involved, using epoll should only improve
things.

One thing that is happening in CGContextDrawImage, on my laptop anyway,
is that the entire image must be resampled in order to account for
the Retina display.