Replacing Private message module with nodes I was looking for a quick answer to the problem of allowing members to write each other messages. The Private message module looked like the most complete solution.

However, it has some significant downsides.


I can't find an API for it anymore. All the links are dead.
It uses awful HTML output which is not responsive so refer to problem 1.
It's not integrated with views!


So I wonder if a poor man's messaging system that simply uses nodes and views would be a more versatile, future proof and potentially more performant replacement. Is private messages dramatically more performant(such that it's downsides are worth it) than using nodes and views for private conversations, of which there would be potentially many?
If by API you mean documentation, there is a documentation for the module in the following link: https://www.drupal.org/documentation/modules/privatemsg
For actual API file: https://cgit.drupalcode.org/privatemsg/tree/privatemsg.api.php?h=7.x-1.x
In the same code repository, the module contains views folder with views integration, so I don't understand what you mean by no views integration?
And here is an example on how to theme the new messages block, you should check this file for more theming functions.
Hope that helps.