Let's say you have a todo app with a million users checking an average of five boxes per day. Currently, you have a simple color shift animation which lasts 200ms and doesn't prevent the user from interacting with other elements. Then a designer suggests a fancy animation popover which takes 3s and blocks interaction, because they read some article about gamification and how it'll increase user engagement.
Disregarding that your app is about providing a specific function and user engagement metrics are meaningless, you implement the fancy animation. Your users are now collectively spending 1,400,000 hours per year waiting for an animation to finish.
Please don't be that person. Your users' time is more valuable than your desire for UI whimsy.
It feels silly to have to type this out, but it's OKAY to have fun sometimes. There's more to life than optimizing every second until you reach the grave.
There's objectively nothing fun about waiting for something which you have no control over and doesn't benefit your life in any conceivable way. You will never find a single human on their deathbed fondly reminiscing about all the checkbox animations they watched. There's more to life than whimsical UI animations.
Disregarding that your app is about providing a specific function and user engagement metrics are meaningless, you implement the fancy animation. Your users are now collectively spending 1,400,000 hours per year waiting for an animation to finish.
Please don't be that person. Your users' time is more valuable than your desire for UI whimsy.