First check the page code for any issues which may prevent fast response. If it runs fine on localhost that is a good sign, but it should also be checked on other computers inside and outside the LAN. If that checks out move on to the web server.
Check the server for bottlenecks, errors, other processes hogging system resources.
If server checks out and the page is being sent to the client in acceptable times, then the problem is in the transport or the client.

Ask the user what kind of connection they are using. Dial-up, broadband, wireless? Is it dedicated to the user, shared with other users? What kind of performance do they get with other web sites/pages? Does the performance change during the day? Are they connecting through a proxy? Is the proxy working properly? Are other users affected?

If the link is okay move on the client's computer. What is the spec of this computer? Does it carry out any other tasks? For example it may be doing some backups or running a report of some sort in the background which the user has forgotten about (or does not know). Is a scheduled virus scan running? What else is the user running - some users open numerous applications and clog the system memory which could be problematic in older/low-spec computers. Find out what browser is being used and if any plugins could be slowing it down?

Somewhere along this sort of analysis you'll find your answer

This is a process question. The interviewer will want to hear a correct answer, but will be more interested in the process you use to get to the answer. Daishik has provided a good process for tracking down the issue.

Remember, there is no perfect answer for this type of question. It is about process.

Think about the process of troubleshooting. Each step should eliminate possible causes until you have a very small set. Then each possibility in the set can be tested until the answer is found.

Hey hey hey, take a gnader at what' you've done

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