I would like my users to answer several questions. They have come to my site with the intent to do this. https://www.typeform.com is a good example. Most of the questions are multiple choice.
Which is best: to switch between questions immediately (<10ms) or have a delay (e.g. 400ms) and fade between the questions (pages).
I can prefetch all possible questions that could follow the current one, so this is not a technical question. Is there any research on this?
It's proportional to the decision making time.
Same principle applies to how complex the question is. For simple "Yes" "No" questions 0.1 - 0.2 seconds (about the time it takes to click the mouse) for others, anything between 0.1 and 0.9 seconds.
I'm not sure about there being research for switching between questions in a form per se, but there is certainly plenty on general page load times which still applies a little in this situation. As you probably already know, the short answer is that people get frustrated very quickly and if they can't do something as quickly and as easy as possible they will move on. So in general it's best to avoid any sort of artificial delays, and sometimes that means cosmetic "sugar" that gets applied to web pages.
My personal opinion on your scenario is from a visual standpoint, some sort of transition between questions is OK as long as it's not over the top. A very quick fade or horizontal / vertical swipe is fine and creates a better experience than just an instant switch. But I would say it has to be quick, 400ms is fine but even extra milliseconds that might be deemed unnecessary can be picked up by a user and can become frustrating.
I wasn't sure from your question if your delay was 400ms and then the fade or if that timing was the actual duration of the fade? If it was the former then I would absolutely shy away from adding any artificial delay to slow things down further. If there were any additional network or system issues from the user's side then you would just be adding to it.
If they have come with the intent, then a fade between each question to make it look pretty does not seem all that bad. This being said, I am assuming that it is a link to a questionnaire that they wish to answer, and have prepared/saved time for. In which case, you may as well make it fancy, but remember, we are busy people (when we are not writing answers on forums ;-) if the fade or transition is too slow it is likely they will get bored and go back to the television. Of course, if urgency is being issued out to your users 'answer by xx/xx/xx to receive 50% off' for example, this may not be an issue.
The best questionnaires I have seen online are the ones that get it out of the way fast. You see the answer, start typing, hit return and the current question swipes out left when the new one enters from the right. These are slick and rememberable. "I wish all questionnaires were that easy" was my response. Sure beats having to click into each individual text box! I think you should ask yourself, do I want this to be remembered because I am awesome at front-end, and it is fancy as hell - or do I want it to be remembered because that was the easiest questionnaire the user will have ever done.
I think this topic is not on trace of "the quicker the better" like most of questions regarding site speed are.
From the point of user, IMO - as long as delay is less than 1 or 1/2 second, it will not cause frustration. As user will not think something is wrong, while "loading effect" wont be perceived.
Also nature of web is that users are used to wait for short time (as most sites were served with data requested and getting back from server).
So sometimes instant response can look pretty unnatural for the users, and they might think something is technically wrong.
What about usability? Users with ADHD/Anxiety and other issues may have trouble with the timing issues
http://www.w3.org/TR/UNDERSTANDING-WCAG20/time-limits.html indicates that you need to
Provide users enough time to read and use content.
For this automatic timing issue, I think this criteria explains in more detail some reasons why people need timing controllable: http://www.w3.org/WAI/WCAG21/Understanding/timing-adjustable.html
People with physical disabilities often need more time to react, to type and to complete activities. People with low vision need more time to locate things on screen and to read. People who are blind and using screen readers may need more time to understand screen layouts, to find information and to operate controls. People who have cognitive or language limitations need more time to read and to understand. ...
However, on that same page, if you scroll down to "Situation B: If a time limit is controlled by a script on the page", there are a few options, based the type of script you are using, to allow the user to opt-out of or alter the delay.
