Shape before detail
Thirty seconds on what it does and who uses it, then the architecture. Opening with the stack turns a deep-dive into twenty minutes of the interviewer reconstructing the problem from your answers.
The test for your opening is whether somebody could draw a box diagram from it. If they could not, it was a list of technologies rather than a description of a system.
Numbers you can defend
Say it scaled and the next question is to what, from what, and what broke first. Say it was fast and the next question is at what percentile.
Bring three numbers you can stand behind and know how each was measured. Three defensible figures are worth more than ten impressive ones, because the conversation is about the one you cannot support.
Name the trade-off unprompted
Every real system has one. Naming it before you are asked is the strongest signal available in this round, because it is the thing that cannot be faked by somebody who only read about the design.
The alternative you rejected and the reason is the answer being listened for. If everything in your design sounds obviously correct, it sounds like a design you did not make.
Being wrong, out loud
A staff-level interviewer will push on something. Defending it harder is the failure; saying what you would change and why is the pass, and it is scored as composure rather than as knowledge.
Priya Raman has built one of these and interrupts when a claim is unsupported. Twenty minutes is longer than it sounds, most people have four good minutes and then begin repeating themselves, which is itself the most useful thing the first run tells you.
