I am editing Ronja Twister
http://ronja.twibright.com/schematics/twister.pcb
And it sometimes takes 30 seconds to redraw (during which PCB doesn't
react). My machine has 1500MHz CPU and 0.5GB of RAM and nothing
CPU intensive is running there besides.
I noticed, after upgrading from an older non-GTK version to a CVS
snapshot from several weeks ago (using GTK+), that sometimes PCB gets
into a funny state where redraws are impossibly slow, as you describe.
After some experimentation I determined that this is influenced by
*other windows* being open on the same desktop, in particular other
maximized windows.