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in newbb preferences, u can set PNG hack to GIF and images to gif instead of png.. that will increase speed slightly in IE altho probably not enough to be noticeable somewhat.
i don't think there's much else that can really be done to increase speed too much.
i'm not exactly sure why, but 1 thing that makes me think could be a cause is the amount of javascript used..
on my old site i had a lot of javascript drop down menu's and mouseovers for navigation round the site, and i ended up taking them off in the end due to a dramatic slow down.
altho i really do like the way newbb has progressed, i do agree compared to other forums it is somewhat slower..
removing un necessary features will increase speed too.. if u really can cope with not having the online user section, then switch it off.. also do u really need to see the HP, XP & MP stats? if not turn them off, same with the show groups option etc..
reducing the amount of topics per page aswell will also help..