Odoo v9 - the first email sent from Odoo was instant, subsequent emails were slow - causing users to experience Odoo as waiting, go get a coffee, etc.
This was even happening when testing the outgoing email server. The first test would respond in 1-2 seconds, subsequent tests immediately after that would take several minutes.
What we discovered was this:
In order to maintain the quality of Amazon EC2 addresses for sending email, Amazon enforces default limits on the amount of email that can be sent from EC2 accounts. If you wish to send larger amounts of email from EC2, you can apply to have these limits removed from your account by filling out the form at https://aws.amazon.com/forms/ec2-email-limit-rdns-request?catalog=true&isauthcode=true
This is also the case when you use Amazon's SES service. You then also have to file a request which is processed in short time and very uncomplicated.