Animations got slower!

I deployed a Phaser 2 game towards the end of 2017 (Phaser 3 wasn’t quite ready at the time I began the project). I literally haven’t touched the code or the deployment since. I just visited the website today, in both Firefox and Chrome, and all the animations are running at about half speed.

I find this super-weird. What could possibly have changed in browsers in the past 3 years to slow down animations in an unchanged Phaser 2 game from 2017?