Trees lag on first pass of camera fly over

Hi

I have a terrain with a variety of tress on it. The scene open with a caema fly over.
I am building for mobile.
When the flyover starts there is a huge lag as it passes through the first set of trees, but if the animation repeats then there is no lag.
I have searched for ways to pre-load the objects but i have come up with nothing (if that is the right approach).
Is there a way to solve this?

Thanks