Sending mails (and using cron with CI)

I am writhing my own user authorization library, and mostly it's going fine (i will share final version on github)

But i have noticed - quite a lag when i sending mail to someone.
For instance - i made a registration form (controller) at the end of which placed mail sending command $this->email->send();
And it can take a while before script is executed (so user see is as page is loading)

is it a good idea to send email in the same script or do i need a make special table in database (pending messages) and just run special "mail script" which is supposed to get all pending messages and send them in the background 
(And run in trough the cron)
And following question - is it possible to use CI (i wanna use CI libraries) with cron? How am i suppose to start it? (because CI start with index.php, not directly from controller..)