Though all the forking of standard CGI makes it far less efficient, which is quite a concern to a cheap shared hosting provider in a saturated market: they need to cram as many accounts as possible on as few machines as possible to have any hope of not running the business at a loss.

Fast_CGI helps here of course, but then you have your users needing to be able to create long lived processes which moves the problem to one of memory use and process cycling & other management.

Yeah, but Plack::Request/Response is a better API to the same for low end stuff and will expand to bigger things as required.