terminology - Defining "Instantaneous" as part of usability acceptance criteria

The Setup

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:

The Question

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).

Answer

The general rule of thumb for usability is to start off with no feedback, but to then display some busy indicator after 200ms, and if the process normally takes 5 seconds or more to present a larger feedback element (usually with a time elapsed timer, but preferably not with a progress bar unless you're very sure how long it will take). If something is likely to take more than 30-60 seconds to complete then you should consider adapting the architecture to support queuing/background processing so the user isn't forced to watch and wait.

If the feedback is meant to be part of an interactive process, like typing on a keyboard and then seeing the keys appear, then you really should be striving to reduce latency/feedback to under 100ms

There's another marker too - it's been found that users make a sub-conscious evaluation of a product in the first 50ms of what they see.

There's another angle to consider too - some accessibility assistive technologies won't see changes to a page if they occur after a delay .. the way they are built is they capture and cache the contents of the page once it is loaded, and then use that cached copy to present to the user. [Specifically JAWS, I can't find the link right now]