An API became slow after a release. Logs show no obvious errors, but requests pile up and some code blocks on async calls. How would you investigate and fix it?

// Released code
var profile = _profileClient.GetProfileAsync(userId).Result;

// Fixed path
var profile = await _profileClient.GetProfileAsync(userId, ct);

The code change is simple, but it may force async signatures through service layers and tests. That migration is the real work.

• What counters would you inspect first?
• How would you prove the issue was thread-pool starvation?
• When would you roll back instead of hotfixing?
• How would you prevent this regression?

This is a strong senior scenario. Listen for evidence-based debugging, runtime tooling, async migration, and rollback judgment.

This checks whether the candidate can handle a real production slowdown, not just explain async syntax.