What is serialisation delay? Why does 10GB Ethernet help reduce it?

Serialization delay is the time it takes to move data from a processor to a network link. Since a 10Gb ethernet link can put bits on the wire ten times faster than a 1Gb ethernet link, the serialization delay will be less.

Think about a typical store-and-forward network switch. The switch must receive a packet, decide which link to send it on, and then send the packet. So the switching latency will be the sum of the time it takes to do each of those three things. The faster the wire speed, the less time it takes to receive or send the packet.

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.