Earlier quoted context omitted.
I think animations, as most program them, slow down the UI exactly as you say. But when their timing is tight, and the whole thing happens in under, say, half a second, you get the benefits of that context association and it still feels really cool and high-tech. People jsut set their animations too slow. (Seriously, nothing should ever take longer than 500ms)
I don't have a link, but in referencing interactivity tests for some remote software development I did some years back, the maximum time between pressing a button and perceiving its effect to be considered "instant" is around 200ms. You don't want to go slower than that if you want it to "feel" fast & responsive.
Now, if that wasn't depressing enough - the Nielsen research is from early 90's. In the intervening 25 years, the users have certainly not become any less impatient.
0: https://www.nngroup.com/articles/response-times-3-important-...