Serialization delay, is the delay needed to get the data serialized. In networking context, that is the delay needed to get the data from the network card, to the cable.
For example, imagine being on a 8000bps (1kBps) dial-up line. If you want to send a 1.5 kilobyte packet, it would take 1.5 seconds to get the data to the line (wire) itself. The minimum delay is 1.5 seconds then. If it's an ICMP ping (reply with same data has to come back), the minimum ping/rtt time is 3seconds. (even though the actual physical speed of the packet is near light-speed).
So on dial-up, it's not just slow bandwidth-wise, but it's also slow latency-wise.
Now back to modern times, even if you wish to send just one packet, it takes longer to send it via 10meg ethernet, then via 100meg, 1gig, 10gig, since it takes more time to get the packet to the wire. Faster line, shorter delay, even if you're always sending just one packet, and don't need the added bandwidth.