Actually compressing doesn't make any difference to the propogation delay since that's the time to the first byte, which remains the same, irrespective of data

Actually compressing doesn't make any difference to the propogation delay since that's the time to the first byte, which remains the same, irrespective of data size. In fact, if compression is performed on the fly, then it will likely increase propogation delay. You're probably referring to transmission time, which is a function of data size.