Your question is Debugging Web Performance Bottlenecks. Take a moment with it on the right.
Talk me through your thinking if you like. When you're confident, submit your answer and I'll grade it like a real screen (7/10 or better passes).
Describe your process for debugging a performance bottleneck in a web application.
Explain how you would isolate the bottleneck, select diagnostic tools, prioritize possible causes, coordinate with relevant stakeholders, and validate the fix. Address how you would manage user impact and decide whether to roll back, mitigate, or continue investigating.