Yeah it's the resolution. GD is pretty limited, and really meant for basic graphical drawing for things on the fly, actual image manipulation is slow and not suggested. Enter ImageMagick.
And yeah Discourse is pretty cool, I remember them from years back. I think we used them for awhile on some sort of hosting deal. Humhub is nice because it's geared towards collaborative discussion, tasks, multimedia (all vastly configurable). The main thing I like is the social network aspect which allows users (if active) to create pages, such as pages for freelancing and commissions, professional work, galleries, their business, teaching and tutorials, etc.