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.
Thanks you and I will head that way when my forum is slow and close it.