Nobody quizzes the table
Nobody quizzes the table for its own sake. They want to watch you justify a design decision with a number instead of a habit.
The move is always the same. State your latency budget, add up what the design spends, and let the arithmetic choose the architecture.
The budget comes first. A user-facing request usually gets 200 to 500 milliseconds end to end before the product feels slow, and your backend's slice is maybe half of that once you subtract the encrypted handshake, the browser's rendering and the last mile. So you are allocating something like 100 to 200 milliseconds across every hop.
Spending the budget
Now spend it, and watch for the trap, which is that sequential calls add up. Five calls in a row inside one data centre at roughly a millisecond each cost about 5 milliseconds, which is nothing. Five calls in a row across regions at 100 milliseconds each cost 500, and your budget is gone before the database is involved.
That single contrast justifies three standard patterns in one breath. Put chatty services near each other. Make independent calls at the same time rather than one after another. Never put a cross-region hop inside a synchronous loop.
A cache is justified the same way, with a number. A database read that touches disk costs around 100 microseconds of storage time plus the query work, commonly a few milliseconds in total. A cache in the same data centre answers in about half a millisecond, and one inside your own process in nanoseconds. If 90 in every 100 reads hit that cache, your database sees a tenth of the traffic.
Any value you forget can be rebuilt from the bands: memory in nanoseconds, disk and the local network in microseconds, spinning disks and long-haul links in milliseconds. Getting the exponent right earns you more credit than reciting digits.
Worked example
Nadia is asked to design a global checkout flow in an interview. She states a budget out loud: 300 ms end to end, 150 ms for the backend. Her first sketch has the US checkout service synchronously calling an inventory service in Frankfurt, and she catches it with her own numbers: 100 ms cross-Atlantic round trip, twice (reserve, then confirm), is 200 ms, already over budget. She restructures: inventory replicates to each region asynchronously, checkout reads the local replica in about 1 ms, and the rare oversell gets handled by a compensation flow. The interviewer pushes back, asking why not just accept the latency. Nadia answers with the ladder: the original design spends two thirds of its budget on two trips across the network, each of which a nearby copy makes nearly free. The feedback afterward specifically calls out the budget arithmetic.