What is the first indication of a performance problem?
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The network delay time is above 15ms
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The DNS is not resolving the machine name
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The Web server's available memory drops below 1 GB
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The end user experiences higher than expected response time
The first indication of a performance problem is when end users experience slower than expected response times. This is the actual user impact. Options A, B, and C are technical metrics that may help diagnose the problem, but they are symptoms or measurements, not the first indication - the user experience is.
The first and most direct signal of a performance problem is that real end users experience response times slower than expected — that's the actual symptom that matters to the business. Metrics like network delay, DNS resolution issues, or low server memory are possible underlying causes that get investigated afterward, but they are diagnostic details, not the initial indication that something is wrong.