Using a cache plugin (all of them, i have tried them all) works up to a point with wpForo.  Several functions do not "show" correctly for VISITORS (not logged), like online users, post views etc etc.

And there is NO solution to that. Either you live with those restrictions, or you exclude wpforo page from your caching plugin.

IMHO wpForo must find some ways to better work with popular FREE cache plugins, like Automattic's WP Super Cache. And to be fair, wpFpro latest versions are much friendlier to caching plugins (apparently wpForo use some WP hooks), still not 100%.