The difference is simulation versus emulation. A post-production blur is a 2D filter that cannot accurately replicate how light, focus, and lens distortions interact in a 3D scene. The OSL shader traces rays through a virtual lens model, resulting in physically correct bokeh, focus falloff, and other optical effects that respond naturally to your scene.

Although the shader performs complex calculations, the performance impact is negligible compared to default DoF.

No. The camera shader requires Open Shading Language (OSL), which is a feature exclusive to Cycles.