I want a decentralized forum more than you, but I'm not going to claim that a decentralized solution (whether it's for a forum or payments) is going to be lighter compared to centralized solutions. I'm afraid you don't understand the technicalities involved.
The burden of proof lies on you...
Decentralized systems undoubtedly carry more "weight" than centralized ones. Replicating data across nodes, running consensus algorithms, coordinating distributed components - it all adds complexity. Perhaps we could develop nimbler decentralized protocols but we should not expect a decentralized system to ever match centralized efficiency.
Ultimately, it is misleading to claim that decentralized systems can be "lighter," or even as lightweight as centralized, and the two approaches feature fundamentally different trade-offs. We choose between them according to the requirements of each application. For some, decentralization provides indispensable security properties, despite the costs and complexities. For others, centralization's simplicity and speed take priority. Blanket comparisons seldom account for those nuances, and BenCodie clearly does not know much about the technical aspects of decentralized networks, or it is possible that he is simply misinformed.