You are correct, that trying to search for these keywords brings up lots of discussion of analyzing performance issues. I did find one, though, that may be relevant. Take a look at this link
from the Postfix documentation. It looks like there are two parameters that can be used to control and insert the delay between messages. One caveat is that they may be oriented towards multiple messages to the same recipient domain. The document is large and contains a vast number of parameters. If these don't do what you would like, try scanning the document for words like delay, time, defer, and so forth. It looks there are some unique items and there may be one that will work. For example look at the default_recipient_refill_delay.