If a client says:

We don't jump in and look at the code and clean it up and reply with something like:

A better way is:

• Ask the client to tell us how slow it is (in seconds) and how fast they ideally would like it (in seconds)
• Add some code to record the time the function takes to run
• Reproduce the steps and record the time
• Change the code
• Reproduce the steps and record the time again
• Reply to the customer:
  "It was 22 seconds, you asked for around 10 seconds. It is now 8 seconds."
  

Figure: Good example – Add some code to check the timing, before fixing any performance issues (An example from SSW Code Auditor)

This is because performance is an emotional thing, sometimes it just *feels* slower. Without numbers, a person cannot really know for sure whether something has become quicker.

Related

For sample code on how to measure performance for windows application form, please refer to rule Do you have tests for Performance? on Rules To Better Unit Tests.