Improve the speed of civimail
I think at some point we did do that. However, we also noticed that some smtp mailers are not very good at reporting back errors and jobs would hang / not deliver email. We put in some code to get around that and periodically check for the connection to be alive and valid

We do check the 'persist' flag in the mailer library.Check

CRM_Core_Config::getMailer

for more details