Mitogen known not to work, and while trying to adopt it, it has shown performance degradation comparing to our in-house connection plugin which handles connection to containers through the host as well (if I'm not mistaken).
And yes, eventually the only way to reduce runtime with current state of the Ansible is reducing amount of tasks, through basically replacing some roles just with custom modules.
But that kinda raises bunch of other concerns/considerations to make.
One of which would be, that code will become hardly understandable for anyone, who is not actively involved in a project development. While currently it is enough for operator to know only Ansible, big amount of custom modules will raise entrance bar needed, including contributions.