Water Pro Shaders (Gerstner Waves) Problem on ios

Hi guys:

I built an example with water , which imported from unity3d's package "water pro". When running on my ipad air, the waves got slowly and slowly as the time past by, but frame rate was fine, about 60fps.

It was weird, so I tried this on macbook, directly running the example at unity editor, and it was all right, the waves were smooth all the time.

Does anybody know?

Thanks for your help.
best regards