Sending is Slow
When it comes to delivery speed, there's a complex combination of limits and performance ability that come into play. In addition to the rates you have set on your third-party SMTP service, and the rates you have set in MailPoet, there is a more basic ability of your website to queue up and send out emails at the chosen rates.
Database and Hosting Performance
When using a third-party sending service (e.g.; Amazon SES, SendGrid), if you set your preferred rate to be 100 / minute on the MailPoet side, what that really means is that the plugin will queue up 100 emails via the Task Scheduler every minute. But in practice, each of these emails requires a database call and some software to run. Actually being able to generate 100 emails per minute depends on your database performance and your hosting package, but we would say reaching such a level of email generation is probably a pretty high ask.
If not all 100 emails can be queued up within the minute, then any emails that were not queued yet are pushed into the following minute. So, say your website and hosting are able to actually generate and push out 40 emails per minute, then the remaining 60 are pushed to the next minute, with 40 new ones requested for a total new request of 100 emails. However, it's likely only another 40 will actually be able to send, and so on, until all emails actually get sent out on a rolling basis.
Basically, setting your MailPoet rate to 100/minute is the idealized maximum, but would be pretty hard to reach in practice.
In addition, the setting on the SMTP service side is also an idealized maximum. They usually state that they may not be able to achieve the chosen rate and can and will send emails out at a lower rate if necessary. You can read more about this here if you are using Amazon SES, for instance.
As you can see, there is a combination of your MailPoet rate with your Amazon SES rate and your website's actual queuing performance ability.
If you are using MailPoet Sending Service the logic is similar. By default, the number of emails per batch is 20, and usually, multiple batches are sent per minute. The MailPoet Sending Service is capable of sending up to 50k emails per hour. However, the actual number of batches per minute depends on your hosting and database performance.
Geographical Location
If you are sending with the MailPoet Sending Service, the communication between our servers and your host may also be impacted by their distance from each other. Our servers are located in Germany, so if your hosting provider is located far away, that may affect the data travel speed.
Task Scheduler Method (Cron)
The task scheduler method you are using might also affect the sending speed. You could look into using Server Side (Linux Cron) instead which can sometimes improve performance