Also, id like to hear what's the reasoning behind using MongoDB for a forum. I mean, I'm just learning this stuff but a forum seems to be a perfect case for a relational DB.
I'll take a shot in the dark and say that the advantage of using a NoSQL platform to host a forum is that you can very quickly load all resources relating to a post by storing them in one database document.
That does mean you probably need to polyfill certain functionality, but the traditional case for NoSQL is made by saying a blog can load a given post's comments, media, etc. with one query. Forums are a similar animal.
NoSQL is great for prototyping and being productive fast, but that productivity comes with a lot of pitfalls. Pitfalls by the way that many devs have been blissfully ignorant of for decades because they have been insulated from them by normalized relational databases.
Unless you are experienced with/aware of those pitfalls, I would almost make a blanket statement that a traditional SQL database should be the default solution for the backend until you can explain why NoSQL is a better fit for a given project.
Also, what makes you think PostgreSQL is slow? What is "slow" to you?