Ask HN: Making our email system faster.

I run a reasonably popular daily deals site (80k members) we were using a managed email provider previously, but over the last week, I decided to take a leap and code our own custom email system built ontop of SendGrid (shout out to SG, those guys are awesome). The difference about this new email system, is we're sending personalized emails to each subscriber, containing only the deals they want to see (we're not a yipit clone, or an aggregator, we run exclusive daily deals in different categories, users can now select which types of deals they wish to receive).

I'm not an amazing programmer, but I'm the lead on this site. Our generation of what to send and where to send it is fine (creating campaigns on the fly, queuing up emails to be sent) the issue arises when sending via SendGrid, currently, we're running a script that opens an SMTP connection to Send Grid, sends a batch of 100 emails (the sendgrid max per connection) and closes the SMTP connection (and repeat). This is pretty slow. We're at about 17 secs to generate and send the 100 email batch, I'm going to do some code optimization on the email generation, which will hopefully speed things up. I'm just wondering if anyone here knows of a good way to speed up our delivery. - As I mentioned, I'm an intermediate coder, what I've built thus far is in PHP/MySQL.