Link utilization impact on responsiveness

Question

Hello,
I'm reading a book, and here it says:That as the link utilization grows (even from 10% to 50%) the responsiveness time grows. I always thought that if the link is ~90proc utilized, then the drops can occur. Can someone explain from physics field, why the utilization till ~50% increases the latency. If we would have a 10gb link and it will be utilized 50% the user will feel bigger latency than the link utilized ~20% ? Why ?
Picture attached.

Answer 1

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The interrelationship between link loading and latency (and drops), and its impact to traffic, is much, much more complex than what that text is explaining.
Without touching on the many, many variables involved, I suspect what the text is alluding to is latency that exponentially begins to increase as utilization approaches 100% due to queuing delays.  Simple queuing theory would, with utilization below 50%, (rarely) should there be more than one element queued.  Once, however, utilization rises above 75%, elements queued will start to skyrocket.  This is what the first graph is showing.
In simple queuing, we normally allow for an infinite queue, but we certainly can decrease possible latency by limiting the possible queue depth.  Of course, in real networks, a high drop rate generally is adverse to most traffic too.  I.e. even though you can avoid high latency, high drops might actually be even more adverse.
If you search the Internet, you can find simple queuing calculators which will, given a certain load percentage, show you what the average queue depth will be at that load percentage.
In any case, again, real world link loading, and it's impact to traffic, is very complex.  Depending on the needs of the traffic, even 30% utilization might be too high while conversely 100% utilization doesn't always mean you need more bandwidth.