This is normal behaviour since your cron triggers only every 15 minutes. It makes no sense to configure your send settings at a smaller interval then your cron interval.
To increase your sending speed you can increase the number of mails in your send settings. If you face a limitation forcing you to send 20 mails every two minutes you need to have a cron job that triggers your acynmailing very two minutes.
The minimum time interval on Acyba's cron server is 15 minutes. In case you need a smaller cron interval you need to setup a task on a different cron server that allows you to run a task at the required interval. This could be don on a cron server at your hoster or any tjhird party cron service.
When you chosse to use a onther cron server you can use the cron url that is displayed in Configuration->License. Please also disable the cron in your Acymailing. This will prevent Acyba's cron server from triggering your Acymailing. Otherwise you would have two cron servers triggering your Acymailin and that will lead to issues.