Do you plan on rolling the coin in any way? You could do a predict physics path on spawn then do a test in the path and see it goes through the terrain on the last point (do a raycast from up above and test if the path goes below the hit point). If the path is below then just cut it at the intersection point and animate the coin along the prediction path.
it will have the correct look physics wise and will probably eat less computational cycles.
The main culprit from short tests is the terrain. Increasing physics substeps to 16 helps a bit bit at steep angles the coin falls through