There are many many factors that determine the speed of sending, and with some testing you can pinpoint what exactly is causing a bottleneck.
Some factors that determine the speed include, but are not limited to; database design, scripting, size of the email (e.g. attachments, inline images, etc), speed of the computer running the plug-in, location of the mail server in relation to the the plug-in, connection speed between the mail server and client, and network congestion or a heavily used mail server.
To determine the bottleneck, it will require testing, more testing, and even more testing. Each test will rule out a possible factor, so in the end, the process is simply trial and error.