If I change any setting in LXAppearance (icon theme, colors, gtk theme, ...), lxsession's process starts using a lot of CPU (about 50% in an atom based netbook). If I close the session and I start a new one the problem disappears. I have tested it on a VM too and the problem is reproducible. It is not that noticeable because the VM runs in a powerful machine, but lxsession uses more CPU anyway.
I'm using the latest stable version available at SourceForge and openSUSE 13.1 x86_64.