How well can Wpforo handle the growth of a website? No problems for smaller sites, but what if it would grow to the size of, say, IGN.com or something like that. Would it be able to handle that traffic without significant load time issues?
wpForo is a Wordpress plugin, so limitations (if any) come also from Wordpress.
So, this is a matter of a powerfull server, mostly, and some optimized Wordpress/wpForo from a specialist. But a so big site, has also the money, so no issues ....
Ok, so if Wordpress is a limitation maybe it's best to run it on invision board or something imilar like that?
A big board, no matter which platform, will require a beefy server.
I guess the benefit of using a stand-alone forum is that you can run it on a subdomain, but then you lose the smooth integration that wpforo offers.
That is one benefit (subdomain). And most of the reputable standalone forum softwares will also offer a bridge to your Wordpress site if it is important to you. Ironically, since the advent of Wordpress - VB, Xenforo IPS etc have developed their own CMS's to satisfy customer demand, thereby making their offerings not so "light" anymore.
And no, I don't find wpForo particularly resource heavy. Sure, the code can be made more streamlined and efficient but that always true of any DB reliant software.
Bottom line, big board, big server, always. And that is what your planning should revolve around initially.
Sidenote: I would not touch VBulletin or phpbb these days.