The Speed Control adds to each newsletter a set of configurations to change the speed (emails per hours) and the allowed delivery hours in the day (starting and ending hours).
Changing the delivery speed, usually slowing it down, can be useful when you promote something but you don’t want a massive access to the site in a short time. For example, if you’re promoting a contest where every day there is a price, it’s a good idea to inform your audience over a long time to avoid the perception that “no one i winning”.
Speed Controls adds even a range of hours in the day where the delivery is allowed (or not allowed). That feature should solve the needs to not send email in the night or out of office hours when the delivery spans over many days (for example if you’re slowing down the sending or if the server cannot manage to deliver all emails in a shot).
Note: the speed and time windows setting are actually available ONLY for regular newsletter, not for Automated or Autoresponder generated emails.