is the forum responsive, mobile friendly? How should I expect forum to impact site speed?
We have several forum themes that are mobile friendly, both free and pro-versions. You can install Simple:Press and test if any of the free ones suites your needs.
Regaring the performance. At simple-press.com we have around 79000+ post in our forum and here is no performance issues due to having Simple:Press installed.
Joakim @ Simple:Press