I reckon we can have a decent decentralized forum protocol if FTTH (100 Mbps upload) becomes the baseline.
With ADSL (1 Mbps upload) it's not really possible... ADSL is fine for BTC, that's why Satoshi released it in 2009 (when ADSL was already becoming the norm).
I believe that a decentralized forum wouldn't necessarily be determined by bandwidth or internet speed. A good solution should be more lightweight, if anything.
I'm afraid you don't understand the fact that a forum has WAY MORE traffic compared to a decentralized payment system exchanging a few numbers here and there.
So no, it's not going to be more lightweight, quite the contrary!