I noticed you are using Eclipse 3.1(M4?), are you also using MyEclipse 3.8.4 Beta for 3.1M4?
Besides that your settings look good… check your log file <workspace dir>\.metadata\.log for exceptions, maybe that will shed some light on this.
ALSO try and create a new Workspace (File > Switch Workspace) name it like “Testing” and then create a new web project and a sample JSP page… does autocomplete work fast there?