It could be clientside rendering times. If you are using standard Parts, then all of the sides of every part are being drawn, which is fine for modern GPUs since they can cull, but mobile phones, or older GPUs – like those in Macs – don’t have the capability of doing so, resulting in a dramatic slowdown.
To fix this, try switching to a MeshPart where the mesh is only a two-triangle surface. That should cut down your render time.
You can also use greedy meshing on the clientside, to merge and resize two blocks with the same exact color to lower rendering time further.