I did not measure agents, but lets take the following example:

Assume you have a 10 GB blu ray movie that you want to watch without waiting for upload/ donwload time.

Maybe you host it on a server on another continent, where you got a 200ms RTT.

And your wire at home supports around 500Mbps - approx 62 MB/s.

Thus to Download it fully, would take around: 161 seconds if you use the full wire and on the happy case.

But to actually watch it and skim through it, you do not need to use the full wire, but around ~11 Mbps for a 2-hour 10 GB film, which is like 2% of the wire. The only hard part is random jumps, as each seek is a full RTT.

If you prefetch too aggresivelly you hide this RTT, but if the wire is saturated, and if you have a cache miss, then you will hang and wait until the request goes through. Might be upt to 1-2 seconds. The whole game is to pace yourself such that you stay ahead of the consumption rate, without saturating the wire.