In firefox or even in text editor geany, switching from current tab to next takes noticeably abount of time - this was't the issue in the xfwm4 (4.6). The rendering of newly switched tab just hangs for about 2/5 second and then renders tab content.
Disabling Compositing does not improve nothing.
The window manager has nothing to do with rendering of applications (unless compositing is enabled).
As disbling compositing does nto improve things, I do not see how this could be related to the window manager.
I took some time to check this on other Linux Distro (gentoo), I compiled xfwm4 with xfdesktop - and everything works as fast as lightning - so the issue is probably limited only to my highly customized ZenWalk distro (and as You said the issue is probably not related to window manager). Sorry for the trouble.
No trouble at all. Should we close this bug?