Is there some industry standard for unacceptable webapp response time?

Acceptable UI response times are based on human psychology and are therefore the same for web applications as they are for traditional desktop applications.

Depending on how the end user perceives the operation that is being performed, an acceptable response time might be 1 second (e.g. for closing a 'dialog window') or 10 seconds (e.g. for displaying the results of a calculation).

The usability guru Jakob Nielsen has written a good article about acceptable web application response times.

Published UI guidelines specify the same acceptable response times, for example:

Java Look and Feel Guidelines

GNOME UI Documentation.

Jakob Nielsen's research has answered this for any application (web apps aren't special in this regard):

• 0.1 second: Limit for users feeling that they are directly manipulating objects in the UI.
• 1 second: Limit for users feeling that they are freely navigating the command space without having to unduly wait for the computer.
• 10 seconds: Limit for users keeping their attention on the task.

So for web apps you should keep your page response times at 500 ms maximum on average near the servers, to have a web app that is a pleasure to use even with a network latency of 200-300 ms.

Yes Nielsen's article has some good info about how psychology is involved. Here you can find more information about why the "perceived performance" matters, and not only the actual response time.