I haven’t been able to easily determine how newsletters are sent using Newsletter. We have about 15,000 people on one of our lists, and if it takes all day to send them their newsletter, this won’t work for us. Do people use 3rd party services like SendGrid, or is there a native sending service in the plugin, or just cron using your server, or what? Thanks
Out of the box Newsletter uses the wp_mail() which can use the hosting mailing system or a service if you have a plugin installed to connect WP with an external service.
Check the speed on main configuration and set it respecting the service you’re using to send emails. For example ask your provider which are their limits.