I'm not comparing modern forums to older forums, I'm comparing both types of forum to OP's minimal forum.

> And even if they had innovated, you can put as big a feature set as you want on the backend and not send 20Mb of JavaScript along with a single request per post and your page will load instantly.

You have to distinguish between initial page loads and subsequent interactions. Forums are not typically one-page-and-done websites: you spend a lot of time navigating around the different views.

For this use case, phpBB forums were and are very slow.

I tested out the phpBB demo [0] and compared it with the Discourse demo [1]. The Discourse demo has a slow initial load, but it's quite snappy after you've downloaded the 6.23mb (not 20mb!) of JS and CSS. The phpBB demo seems faster on the initial page load, but it gets pretty painful to navigate as you click around into different sub-forums.

The reason why the OP's no-JS forum can outperform both phpBB and Discourse is primarily that it doesn't do very much work on each user action, not that its stack is inherently superior. The biggest bottleneck in a forum will be the speed of each request-response cycle, not whether the requests return JSON or HTML.

[0] http://www.try-phpbb.com/33x/index.php

[1] https://try.discourse.org/