When using Soapbox Mailer, all of your email is being delivered by the good folks at Amazon via their Amazon Simple Email Service (SES). They have a few different quotas (limits) on their services, based on your account status with them. One of the key quotas is your email sending rate (or speed of sending emails).
Amazon SES sets your sending rate based on the number of emails per second they will send on your behalf. Want to find your Amazon SES sending rate?
1) Go to the Amazon Web Services console: https://console.aws.amazon.com/console/home
3) Click on the link to SES: https://console.aws.amazon.com/ses/home
4) Look at "Your Amazon SES Sending Limits", specifically "Max Send Rate"