After a couple of seconds (usually less then 15~), thunar starts using 100% disk IO and makes the entire system, including itself unresponsive.
strace suggests slowdown comes from repeated calls like this:
writev(3, [{"&\3\2\0\3\0`\3", 8}, {NULL, 0}, {"", 0}], 3) = 8
poll([{fd=3, events=POLLIN}], 1, -1) = 1 ([{fd=3, revents=POLLIN}])
read(3, "\1\1J\v\0\0\0\0Z\1\0\0\0\0\0\0\313\4%\3\312\4\371\2\20\0\0\0\0\0\0\0", 4096) = 32
read(3, 0xc680f4, 4096) = -1 EAGAIN (Resource temporarily unavailable)
poll([{fd=3, events=POLLIN|POLLOUT}], 1, -1) = 1 ([{fd=3, revents=POLLOUT}])
writev(3, [{"(\3\4\0\3\0`\3Z\1\0\0\312\4\371\2", 16}, {NULL, 0}, {"", 0}], 3) = 16
poll([{fd=3, events=POLLIN}], 1, -1) = 1 ([{fd=3, revents=POLLIN}])
read(3, "\1\1K\v\0\0\0\0\263~\363\1\313\4%\3\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0", 4096) = 32
read(3, 0xc680f4, 4096) = -1 EAGAIN (Resource temporarily unavailable)
read(3, 0xc680f4, 4096) = -1 EAGAIN (Resource temporarily unavailable)
read(3, 0xc680f4, 4096) = -1 EAGAIN (Resource temporarily unavailable)
poll([{fd=4, events=POLLIN}, {fd=3, events=POLLIN}, {fd=7, events=POLLIN}, {fd=8, events=POLLIN}, {fd=9, events=POLLIN}, {fd=6, events=POLLIN}], 6, 1838) = 1 ([{fd=3, revents=POLLIN}])
read(3, "\2\33K\v\31\210&4Z\1\0\0\4\0`\3\0\0\0\0\313\4%\3\313\4\372\2\20\0\1\0", 4096) = 32
read(3, 0xc680f4, 4096) = -1 EAGAIN (Resource temporarily unavailable)
poll([{fd=3, events=POLLIN|POLLOUT}], 1, -1) = 1 ([{fd=3, revents=POLLOUT}])




not completely sure but the 3 fd might be:
connect(3, {sa_family=AF_FILE, path=@"/tmp/.X11-unix/X0"}, 20) = 0
anyhow, the entire strace log is given

pacman -Qi thunar
Name : thunar
Version : 1.0.2-1
URL : http://thunar.xfce.org
Licenses : GPL2 LGPL2.1
Groups : xfce4
Provides : None
Depends On : desktop-file-utils libexif>=0.6.19 hicolor-icon-theme fam
gtk2 exo>=0.3.107 libxfce4util>=4.6.2 hal>=0.5.14 libpng
libjpeg>=8.0.1 startup-notification
Optional Deps : gamin: a fam replacement to make thunar monitor changes to
files on the fly
xfce4-panel: for trash applet
Required By : thunar-archive-plugin thunar-audio-thumbnailer
thunar-media-tags-plugin thunar-vcs-plugin thunar-volman
Conflicts With : None
Replaces : None
Installed Size : 15380.00 K
Packager : AndyRTR <andyrtr@archlinux.org>
Architecture : x86_64
Build Date : Sat 22 May 2010 04:15:41 PM CEST
Install Date : Fri 08 Oct 2010 07:48:22 PM CEST
Install Reason : Explicitly installed
Install Script : Yes
Description : new modern file manager for Xfce

i'm putting this as critical as it's been around for a while and makes me have no decent fm on linux :(