When CiviMail renders smarty templates for mailing, it saves them to disk

We regularly send mailing to almost 200,000 people and have had all sorts of performance problems. We noticed that for each contact, the smarty template gets compiled and then cached in templates_c. At one point we actually almost ran out of disk space because of this.

I think if I make it so smarty templates that get compiled for mailings never touch the disk (since they're only used once anyway), it might improve performance a whole lot.