You can specify how fast (or slow) you want to send emails in Kentico if you use the email queue. You can batch emails being sent into 10 or 100 or 10000 if you'd like.
Essentially what's going to happen is when the newsletter is sent, it will create an email record for every subscriber with the specific content. So if you have 10,000 subscribers, all of those emails will be created and sent to the email queue. Then the email queue processes the queue based on whatever you have specified in the batch setting. So if you have 100 as the batch, the first 100 will be processed. Then the next time the email queue is ran/processed, the next 100 will be sent.
You can also modify the scheduled task that process the email queue to run at different intervals (say once every minute vs. once every 15 minutes).