How fast does Avalon need to be? For anyone looking at the new drawing subsystem (code named Avalon) in the next generation of Windows (code named Longhorn) this is an interesting blog by one of the architects. Along with general goals they have in mind for Avalon... he also talks about generic usability guidelines for speed in software:

An interesting guiding article comes from the useit site. Jacob Nielsen defines 3 important limits for systems of all kinds:

• Anything between 0-.1 seconds is fast enough for users not to be distracted from their task.
  
• Between .1-1 second is tolerable, with sufficient feedback. However, users’ attention may wander away from their task.
  
• Between 1-10 seconds, users’ attention is distracted from the task. Users might not wait for the download, and cancel out.
  

A collection from various usability engineers:

• Faster is always better
  
• Consistent response times are better than inconsistent ones
  
• Speed affects not just the perception of performance, but also the perception of the resulting content. For Avalon this means eye candy needs to be balanced with proper speed. [Ramsay, J., Barbesi, A., and Preece, J. (1998) A Psychological Investigation of Long Retrieval Times on the World Wide Web. Interacting with Computers, 10, 77—86]
  
• Smoothness of animation and transition are very important
  
• And the most basic rule: Be faster than your competition for comparable scenarios