Each time a Sequence starts (Manually or by a Schedule) or when follow-ups gets due; emails are added to the send queue of your inbox. Follow-up emails are prioritized over initial emails.
That means no initial email will be sent out until all pending follow-ups have been sent out.
This is to ensure that follow-ups won't accumulate to unmanageable levels.
You can view the Send Queue from the Email Inbox's Settings page -> under Action -> View Send Queue:
Emails are sent out one by one depending on the minimum time delay you've set -> How Quickmail Sends Emails
QuickMail will then process the Send Queue at the speed defined by the delay. This applies to both initial touch emails AND follow-ups (changing the delay affects the queue immediately)