Slow animations are a way to hide latency, they are essentially loading screens. Apple is really good at it, or at least it was with the early iPhones, and a reason why iPhones felt so smooth compared to their Android counterparts while not being actually faster. For me, it is an impressive technical feat and it took years for Android to catch up (see: "project butter"), and in the end, it was mostly by brute force, i.e. putting ridiculously overpowered hardware in smartphones.
Remove the animations or make them faster (you can do that sometimes), and the lag may become apparent.
Why you have latency to hide in the first place is another problem. There may also be some clueless designers who put slow animations for no good reason, maybe because they are just copying Apple, not understanding why Apple did it in the first place.