According to the docs for gammu-smsdrc(5):

ReceiveFrequency

The number of seconds between testing for received SMSes, when the phone is busy sending SMSes. Normally a test for received SMSes is done every CommTimeout seconds and after each sent SMS.

Unless I'm misunderstanding something, every time a message is sent, the service checks for received messages. Surely this hurts performance when sending mass texts. It'd be preferable to check only every couple of minutes (or whatever one sets ReceiveFrequency to) instead of constantly interrupting the send process.