Velocity Session on Psychology of Performance

Psychology of Performance

Stoyan Stefanov (Yahoo! Inc)
2:30pm Wednesday, 06/23/2010
Velocity Culture, Web Performance Ballroom AB

Topics of discussion:

  • Relativity of time
  • Mind hacks and perception
  • The state of flow
  • Considering the user expectations (competition, page purpose)
  • User-onload time
  • Providing feedback to the user
  • Using transitions
  • Progressive rendering
  • How long is a blink of an eye
  • Color and speed
  • Clutter

People planning to attend this session also want to see:

  • Stupid Web Caching (and Other Intermediary) Tricks
  • Building Performance Into the New Yahoo! Homepage
  • TCP and the Lower Bound of Web Performance
  • A Day in the Life of Facebook Operations

Stoyan Stefanov

Yahoo! Inc
Stoyan Stefanov (, @stoyanstefanov) is a front-end engineer at Yahoo! Search. Creator of the online image optimization tool and architect of YSlow 2.0. Book author (Object-Oriented JavaScript) and contributor (Even Faster Web Sites, High-Performance JavaScript) and speaker (Velocity, Ajax Experience, JSConf).

Notes on the Session

Started with a little video about waiting for the computer download. The elapsed time actually increased. The link to the move is here. Started by talking about perception and how it affects performance. Surprisingly, Stoyan’s session was almost as big as the Facebook session. Only difference is that he’s not that confident of a speaker so he struggled a bit.


  • Actual
  • Expected
  • Perceived
  • Remembered

Showed a simple chart demonstrating the 4 types of durations over time.


  • Visual Illusions
  • Temporal Illusions

Made a big point that animation sucks. I agree, but what options do we really have?


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