There are many factors here that may have nothing to do with the perl modules you've chosen.

What sizes are the message, whats the connection setup time with your mailserver, could *it* be the slow point ?

Net::SMTP may be more lightweight, but I think you need to do measurements with both and a standard set of message mixes to know for sure. Also define your expectations. How quickly would you like one message, or ten, or one hundred sent in one go, to arrive in the MTAs queue ? Compare this same set of messages to echoing a prepared SMTP session directly to the MTA server port 25 using netcat, and see how much overhead the server accounts for, independent of your perl script or modules.

HTH

Alan