Speed and bandwidth are concepts often confused to a point where both measurements seem interchangeable. They are not. Here is how speed and bandwidth are described by a Cisco network engineer (Bandwidth vs. Speed | The Cisco Learning Center | Chandan Singh Takuli):
“Too fast Too furious – who doesn’t like speed, especially when we talk about the internet or network connectivity? But the real question is, which is better to have: fast speed or more bandwidth? Although these terms are inter-related, they’re not same. As an internet or network user, “fast speed” means a faster rate of data communications. That sounds good, because who doesn’t want a fast network connection? But when we start thinking about it as network engineers, things change a little bit as we talk about bandwidth over WAN and speed over LAN. Many network engineering friends of mine ask me, “What’s the difference?” So let’s dive into it.
The data traveling speed over media is a different concept than the speed of network we are talking about here. When we say “high speed network,” we are not talking about data signals’ traveling speed over network media, but we are talking about data transfer speed or rate across the network. Seem a little confusing?
Let’s look at an example of water flowing through a tap. If a bucket can be filled with water from the tap in 5 minutes, that means we can fill 12 buckets of water in 1 hour, which gives us a rate/speed of 12 buckets/hour. Now if you double the width of the tap pipe and mouth, you will notice that the time taken to fill a single bucket is shortened by almost half and we can fill 24 buckets/hour. So our rate is doubled. (Remember that the water is flowing at the same speed inside the pipe as it was earlier.) The same concept applies to networking: the tap pipe is your link or media, the width of the pipe is your bandwidth, and the water is your data. The rate of data transfer depends on many factors, among which bandwidth is one of them.
Bandwidth is the capacity and speed is the transfer rate
More bandwidth does not mean more speed. Yep, you read that right. Suppose you have double the width of the tap pipe, but the water rate is still the same as it was when the tap pipe was half as wide. It will not result in any improvement in speed. When we talk about WAN links, we mostly talk about bandwidth; when we talk about LAN, we mostly talk about speed. This is because we are most limited by costly cable bandwidth over WAN rather than hardware and interface data transfer rates (or speed) over LAN.”