Okay it might be trickier than I thought. The screen with the user list and the details page are two different elements. When you click on a user, it adds display:none to the user list and remove display:none from the details page. And the other way around when you click on the Back button.
I thought of adding a CSS transition property to each of the two elements, but that can’t really work because browsers disable CSS transitions when you change the display property. Unfortunately, I don’t think it’s currently possible to change the speed of the animation. That said, in my opinion, the animation looks good as-is in your app.
Also while doing my research, I found a few other discussions about similar issues. You might be interested in this and that. Sorry I couldn’t be more helpful…