[gnome-flashback] Main menu 10 times slower in Ubuntu 12.04 vs 10.04

[Alkis Georgopoulos <alkisg gmail com>]

A video of gnome-panel menus in Ubuntu 12.04 (1:3.4.1-0ubuntu1) going 
incredibly slowly, able to draw only about 2 menu items per second:
http://www.youtube.com/watch?v=aEoe2LlrY-I

Doing the same thing in Ubuntu 10.04 (1:2.30.0-0ubuntu1) shows that the 
menus there were 10 times faster:
http://www.youtube.com/watch?v=LZGW2eJEHvY

Those are on the same PC, Pentium 4 at 1.7 GHz with 1 GB RAM.
I tried several graphics cards, it's not related to the graphics card 
drivers.

So, somewhere between those 2 gnome-panel versions (maybe the gtk3 move 
was involved?) a huge regression happened, and now the menus are almost 
unusable in older PCs.

It's possible that the problem is not in gnome-panel itself but in some 
of the involved libraries, or the themes, or even the gcc compilation 
options, but does anyone have some idea what would be responsible for 
this and how to pinpoint it so that we report it to its developers, _if_ 
gnome-panel is not to blame here?
I didn't notice (yet) any significant regression in other applications.

Thanks,
Alkis