First, please keep in mind that anything I say here is my experience and my opinion… Ask 100 people about wp caching plugins and you will get many, many different opinions… and my opinion is irrespective of simple press and from my experience on using with other sites…
If you are not using some sort of opcode caching (ie memechached, apc, etc) with w3tc, its probably not helping your site but may be hurting… yes, a caching plugin can hurt your site performance… I have no idea if this is the case for you site, but 5 seconds normal response time is pretty poor to begin with… I have tried w3tc, super cache and other plugins on many of my sites, and generally speaking have seen little to no improvement…. your mileage may vary of course… so when I say configured properly, just meant have you run some performance testing to even see if its helping or hurting your site – I would not assume automatically that its helping…
as to how to disable it for the wp page the forum appears on, on each of the different cache types of w3tc (ie page, db, etc), there is an exclusion or ignore list that you can set… you need to add your wp page the forum appears on to that exclusion list… the caching plugin incorrectly assumes that the wp page the forum appears on is static in content.. it is NOT… all the appearance of many forum pages is simply url rewrite black magic and all the display is on that single wp page… so you cannot cache it reliably…