SDE Skills volunteers have run and recorded many system design mock interviews, each followed by an open feedback round. The same handful of points come up again and again. Here they are, with links to the exact moments in the recordings.
Strong candidates narrow the problem early and say their assumptions out loud: one user’s devices only, a few cities instead of the whole world, one sport instead of every sport. A smaller, clearly stated problem leaves time to go deep. (example at 3:25, another at 1:47)
In one feedback round the reviewer timed the interview: about 8 minutes on estimation, 13 minutes before reaching the core problem, and 43 minutes before any discussion of scale. That left almost no time for the part that matters most. A rough budget for a 45 to 60 minute round:
| Phase | Time |
|---|---|
| Requirements and scope | 5 min |
| Rough estimates (only what changes the design) | 3 to 5 min |
| High-level design | 10 to 15 min |
| Deep dive on the hardest part | 15 to 20 min |
| Scale, failures, wrap-up | 5 to 10 min |
(the pacing feedback at 62:24)
Aim to talk around 60% of the time and leave room for the interviewer to steer. Before time runs out, walk the final design back through the original requirements so the interviewer sees every one covered. (feedback at 53:49)
Interviewers ask “why is this here?” about each component. Extra queues, caches and services that are not needed count against you. In one session a candidate’s own verdict on their rate limiter design was that they had over-complicated a simple problem. (63:04, 42:23) See our rate limiter guide for a right-sized answer.
It should feel like two engineers discussing a design. If you already know a question, say so rather than pretending to work it out from scratch; interviewers notice. (64:16)
Pacing and structure feedback
Talk ratio and mapping back to requirements
SDE Skills is a volunteer-run 501(c)(3) nonprofit. Join a free weekly system design session on Zoom, or ask in our Discord to pair up for a mock interview. See the schedule and How to Prepare for a System Design Interview.