Create model LOD so shadow will be cast from that LOD, by default shadow will be casted from the 1st LOD, if you have more than one LOD, you can switch the shadow_lod to higher level of LOD(lower poly). Some artifact will appear if the shape difference is too high though.

If your model doesn't have any LOD, shadow will be cast right from the model, even if it has high poly. And of course it will kill your fps.

And I don't think if shadow's alpha will affect rendering speed

Yes, LOD helps with stencil shadow speed. The other thing that helps is accelerated shadows, make a test with the A8 free version. I think A8 supports accelerated shadows already in the free version, with A7 it was only in Pro. Accelerated shadows are much faster, the fps is 2 or 3 times higher.

Thanks for the advices, i will try something with LOD.

Spirit, did accelerated shadows work with c-script and A8?

They are always activated in A8, so they work also with C-Script. But the free version does not support C-Script. Only Extra or better.

Thanks Spirit, first i will try in trial how this work