Investigate Mac event handling slowness when using many widgets in a scroll view.

Looks like we are hitting a slow code path somewhere inside Carbon:

"The big5 example application simply creates a lot of QWidgets (simple,
fixed-sized, random-colored) squares in a big view hierarchy inside a
QScrollArea. Note that after the app finishes opening its window under
MacOS/X (v10.4.8), the window is small, and CPU usage hovers at around 40% –
I would expect CPU usage to be around 0% since the app isn't doing anything.
Also note that simply moving the mouse pointer around inside of the app's
window increases CPU usage significantly (often to 100%).

Now make the app's window as big as possible (either by clicking the green
button in the window's title bar, or by sizing the window larger with the
mouse). Note that once the window has been expanded to its maximum size, the
app's CPU usage goes down to zero – that is to say, making the window large
enough to display the entire contents of the QScrollArea clears the fault.
The symptom does not re-occur, even if you make the window smaller again."