If you import 60fps into a 30fps, the frames will get turned into fields. You will have 30 fps with 60 fields, 2 fields in each frame. Your eyes won’t see it though. that’s almost exactly what televisions show.
If you’re using timewarp with fields, you have to place it in double the footage fps comp so that timewarp can access both fields again.
If I were you, I would speed up the footage inside a 60fps comp then place it as a precomp in a 30fps. That way timewarp would work well and not be smeary.