Ok so I found what's causing the slowdown - the fact that I have a progress bar in my plugin!!
Whether I create it as a TaskProgressBar or a RangeProgressBar, it being in existence absolutely kills the ExportImageToPNG function.
I'm guessing this is because something under the hood seems to hook into the progress bar, adding the text "Writing PNG" just above the bar - this hasn't come from my code.
Surely this is a bug in CC? The same "Writing PNG" message also appears in CS5.5 and 6, but there's no slowdown in those versions.
Some feedback on whether this is a bug or whether I'm doing something to cause it would be much appreciated before I go off and spend time writing my own progress bar...
Thanks