The perfect duration for CSS transitions

We have all experienced a website interaction that feels sluggish or otherwise off on account of poor transition or animation duration and timing. However, there is a very simple "golden rule" to help you avoid this poor user experience, called Doherty Threshold:

Productivity soars when a computer and its users interact at a pace (<400ms) that ensures that neither has to wait on the other.

The magic number of 0.4s sounds like a very reasonable threshold, but take a look at any of your favorite websites and you'll notice that most transition-duration or animation-duration values are closer to 0.3s. This might have something to do with our more recent expectation of what a fast interaction should feel like - after all the relevant research paper was published in 1982.