With JunQuilla installed on Shredder, navigating through messages in the thread pane is comparatively extremely slow. I had filed a bugzilla.mozilla.org bug, but it got marked invalid, suggesting I report the issue to extension devs. I'm on a P3 933 MHz.
Just discovered that the problem occurs only after clicking to "Remove" Uncertain folders then clicking OK.
So the extension appears to be messing something up. Once this happens The only way to get the performance back is to disable the extension and restart.
Next time the extension is enabled, performance is still okay until "Remove" Uncertain folders is again clicked; then the problem returns.
p.s. the remove Uncertain folders option is not persistent and those folders keep showing up on every restart of Shredder.
I tested on mine, and I did not see the performance problem when I removed the Uncertain folders – but I did see the problem that the remove does not persist. I'll do some more investigations.