WPF performance issue when resizing the window with lots of controls

I have a WPF window that contains a fancy image with roughly 200 controls (derived from buttons), all of which use one of my 5 templates (paths, shadow effects, etc). Agreed, it is a heavy window to draw. I can live with that.

My problem comes from resizing the window. Maximize/Restore take about 1-2 seconds, but manually dragging the bottom-left corner causes the system to hang for about 5-10 seconds. In that delay, the window is black & contains partial leftovers until the final result is shown. It looks amateurish and that, I can't live with.

Remote connection : using a remote account, I found that the window resize always takes 1-2 seconds, but doesn't draw the "intermediate" stages while I'm dragging the window borders. The result is as snappy as I would expect.

My conclusion is this: It's the redraws during the resize that are bottlenecks.

The inevitable question is this : how can I prevent redrawing the window until the resize is finished?

Thanks in advance for any ideas...