Performance / Latency Tail Interview Questions 2026

Design a connection pool for database connections. What parameters would you expose and what are the failure modes?

Your gRPC service has p99 latency of 1 second. Adding a timeout of 500ms seems to help p99 but now 1% of requests fail with DeadlineExceeded. What's the correct approach?

An engineer proposes adding an in-memory cache to every service to reduce database load. What are the problems with this approach at scale?

You notice your database CPU is fine but disk I/O is at 100%. Queries are fast individually but slow under concurrent load. What's happening?

Your API response time is 200ms. You add Redis caching. Response time drops to 20ms for cache hits but is now 400ms for cache misses (double the original). What happened?

You're building a feature that requires reading from a database 1000 times per request (for each item in a list). Batching is not possible due to the library you must use. What do you do?

An iOS app's scroll performance drops to visibly janky frame rates on a list with complex cells. Walk through your process for diagnosing whether the bottleneck is layout, rendering, or data processing.

A web-based PDF viewer becomes unresponsive when rendering a 200-page document. What rendering strategy would you use so the app stays responsive?

An app's cold launch time regressed from 800ms to 1.4s after a release. What's your systematic approach to finding the regression before assuming which commit caused it?

A microservice that was handling 500 req/sec now crashes at 100 req/sec after adding a new feature. The feature adds one line: logging the full request body. What's happening?

Your team wants to add caching everywhere to improve performance. What questions would you ask before adding any cache?

You're asked to reduce your service's AWS costs by 40% without degrading performance. Where do you start?

Your API's p99 latency is 3 seconds but p50 is 50ms. Most users have a great experience but 1% see extreme slowness. How do you investigate the outliers?