I have run into this problem time and time again. An interface is performing slow and QA and/or Engineering come back and ask, "can you please define the acceptance criteria for how fast it needs to perform."
Often the answer is (or should be) Instantaneously, however this is often met with, "well we need a number." or similar comment. I agree instantly is up to interpretation, may be unhelpful, etc. However, me picking a number out of thin air like 50 milliseconds or 250 milliseconds is just as unhelpful as QA will have no way to validate this.
It usually comes down to the old adage "I'll know it's right (or wrong) when I see it"
So my question to you is:
How do you define "instant response" as part of your usability acceptance criteria when speaking about performance. What has worked, and what has not worked. If you did set a exact number, how was it tested or confirmed? What is instant reaction time?
An example situation
You have a grid that is editable and need to define how long it should take when you click to edit (how long till the cell displays as an input) and how long it should take to commit the change when exit the field (how long it takes to turn into or appear as plain text).