You can put your forum in
debug mode.
On each page it will tell you how long it took to generate the page... You can then confirm if the index really is taking a lot longer or not. It could be some kind of 3rd party/external content slowing your page.
If debug mode does show the page generation taking a long time it will also tell you if you have any un-cached templates (they'd be in red) and other possibly valuable info.
You should close your forum before turning on debug mode.