Light Optimization for Mobile

Hi, I'm working on a 3D checkers game in unity. I have pieces with 310 vertices and in total 24 pieces in the game screen. They share the same material. When I use directional light without shadows I get fps slightly lower than 60. However for better looks I want to use shadows and If possible spot light instead of the directional one. But when I use spotlight with or without shadows fps drops significantly. Directional shadows doesn't cost as much but their shapes are abrupt. Whether directional or spot I use mixed lighting with baked global illumination in shadowmask mode. I want to know if it is possible to use spotlight with shadows when pieces have that much vertices. If not is it possible to fix the shadows of the directional light, they look spiky.