how is it possible that a bird in flight is slower than a crippled old man and a giant rock?

She's really cold and it's not easy for her to move around or cast spells very fast. That's what happens to me when it's really cold outside!

Because Malphite is moving as fast as he can.

plays an instrument she should be flying at the speed of sound.

Because logic is inferior to balance and champion coherence here.

Anivia is a ice bird. Ice = cold = everything moves slower (even atoms - or especially atoms and molecules).