It could be several items such as mysql is not tuned properly.
There is a recent post the last few days on this.
Also you may need to optimize your tables. This could be done through
Control Panel » Database Tools-Information
Also, If you have not done so is turn on "Enable zlib Compression" in the cp>>Primary settings-Advanced options.
Another item Rick noted in the past with large boards is to turn off "Show total # of unread posts per forum"
Control Panel » Display Options-Forum Summary Options .
If I had to guess what the problem is I would guess it is the 100 forums with "Show total # of unread posts per forum" enabled.
You can read about it
HERE
