I've got a similar issue. I posted my overnested code example at the Migration thread first.
I've got an overnested panel with multiple checkboxes. I tried to rebuild a similar example. Created some random Panels including some checkboxes.
With Ext 3.3.0 (ext-all-debug.js and ext-base-debug.js) it will be rendered within 300 up to 400 ms.
Ext 4.0.0 needs more than 40000 ms to render the same configuration in Firefox 4.0.1.
It seems to me that the layout function of Ext.layout.Layout needs a lot of calculation time. Especially Firefox asks at the Line of "me.onLayout.apply(me, arguments);" to stop the script.
[edit: FF4 wants to stop also at the getWidth() method of the Ext.core.Element line 8485 in ext-all-debug.js].
Most of the componentes are using the standard layout.
With using more hbox layouts Ext 4 will render a little bit faster.