If some window is foolish enough to set an incredibly long window title, then Fluxbox will slow down a lot when rendering the Iconbar (and maybe the Window Titlebar too).
You can make your browser's title very long by doing this in the browser console (Chromium here):
document.title = new Array(4999).join(".");
Now try switching desktop, or Alt-Tabbing off and back onto the window, keeping your eye on 'top' or 'gkrellm'.
4999 was enough to hang up Fluxbox for a few seconds on my 32bit 2GHz machine, adjust as required.
You may need to set the IconbarMode to "Workspace".
A simple fix would be to temporarily trim the window title down to ~300 chars before rendering the text (in the iconbar or the titlebar), if it is longer than 600 chars. (Perhaps also append "..." to indicate this has happened.)
Thanks. Best window manager ever!! :D