How do you debug slowness and bugs?

The interviewer posed two applied engineering questions:

1. A web app's frontend loads very slowly. Walk through how you would diagnose the problem from end to end, pinpoint the exact bottleneck—whether a specific component, asset, API, or DOM element—and then make performance improvements.
2. A production bug has been found. Explain how you would judge severity, limit the impact, investigate the root cause, update stakeholders, ship a fix safely, and stop it from happening again. Overview: This question tests troubleshooting and incident-response skills, including performance debugging, root-cause analysis, severity assessment, mitigation and impact reduction, stakeholder communication, and safe deployment practices for web applications. Read the full software engineer interview experience this question came from.