[calorie]

Okay, thanks for the update. Not sure if you know, but the forum posts count isn't a piece of information that gets cached. If page text is cached, the load can be smaller because the page text wouldn't have to be HTMLized. For uncached page text, the text gets HTMLized and that can increase load, but the forum posts query happens regardless, page text cached or not. Not sure if you are using debug mode, but if not, maybe try sticking the following in the config.php file, load a page, and follow the explain link. It'll show you stats on how the forum posts query performs, but it's subject to response variability. If you're up for it, try comparing several cached and uncached pages with explain, picking pages with about the same number of BB codes and images. It'd be interesting to see how the forum posts query performs on a board of your size.

// change xxx.xxx.xxx.xxx to your IP address

if ($_SERVER['REMOTE_ADDR'] == 'xxx.xxx.xxx.xxx')
{
        $config['Misc']['debug'] = true;
}