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Picture it this way
There is only one counter. It is the only place where work actually happens, and while a customer is away at the vault fetching paperwork, that counter stands empty.
With a single customer, that one person works through their errands in order — and every trip to the vault leaves the counter idle. The waiting from the last chapter shows up here as an empty counter.
So what if four customers are in the bank at once? While one is at the vault, another is using the counter. Nobody built a second counter; the empty time simply got used.
Switch "customers at once" between 1 and 4 above. In a computer, one of these customers is one thread.
Now the numbers
measured · computed just now
One customer working through 12 errands alone takes 96.0 ticks to finish them all (a tick here is the simulator's own clock, not a second — what matters is the ratio between the two numbers). Over that whole stretch the counter was actually working 25.0% of the time. The rest was spent waiting for paperwork to come back from the vault.
Raise it to 4 customers and the same 12 errands finish in 34.6 ticks — 2.8 times faster. The counter is still a single counter; only the share of time it spent working went up, to 69.4%. The number of people at the vault at once went from 1 to 4.
It is not free. The number of times the person at the counter changed rose from 0 to 21, and the memory held open at once went from 1 place to 4 — each customer keeps their own. Keep adding customers and this cost is what eventually sets the limit.
These numbers are not written into the sentence. They come from a simulation run while this page rendered.