Veikkaus, the Finnish national betting agency, got people to add more items to their online shopping cart after they made their UI animations and transitions faster. Why would faster transitions result into bigger shopping carts?

To answer this question, we need to think about the way we (the user) interact with websites and applications (the system).

The user has some kind of task they want to complete with the system in hand (for example, adding items to the cart). How will the user start interacting with the system? How will the user decide what actions to take?

The traditional way of thinking about these interactions is a loop. The user does an action, the system gives a response, and based on that action the user does the next action.

In reality, we plan a lot more ahead as users. Often the user already has a sequence of actions in mind. We basically assume how a system will work and what is required from us. We don’t stop to think after every system response. Instead, we enter into a flow where we reflect less and do more. System responses assure us that we’re on the right path. They direct us in small ways. Or they stop us when necessary.

You can probably see how we interact faster with systems in this actions-as-sequences kind of way compared to the action-response-think loop.

In 1979, IBM researchers Walter J. Doherty and Richard P. Kelisky suggested that increases in system response times disrupt our action plans. When the system response time increases, it breaks our flow and we get into the traditional loop where we actually reflect on the responses we get and question our planned action sequence. The rate of our clicks and taps start to slow down.

On the internet, you might run into “Doherty’s Threshold”, named after the aforementioned IBM researcher. Doherty’s Threshold dictates that a user experience turns from painful to addictive after the system feedback time drops below 400ms. This magic number of “400ms” is BS. The number is probably invented by the writers of the TV show Halt and Catch Fire where the term was first introduced.

What’s true is that we interact faster with systems that give us fast feedback. And remember, transition time is part of this experience of speed.

If you still want magic numbers to replace Doherty's Threshold, here’s some for you: Our brains finish registering images in 100ms. Our average reaction time is 250ms. Google’s Material Design suggests that simple animations should take 100ms to complete, more complex ones 500ms.