Hi @jackvinson
Most people would want to slow down the sending of emails in phpList so that the emails are received and put in the inbox of the recipient.
Although there are no limitations built into phpList, the overall performance depends on many things, hence the list of factors above by @NYChris.
There are many factors that exist outside of phpList that will lower the throughput. For instance, if your email server is configured to send via 200 simulatneous smtp outgoing connections, and 50 of them are locked up waiting for AOL, that would lower the performance.