If you get bad rendering on a big curved flat surfaces try this... Open the surface. Here I deleted one point in the middle and all connected faces were radially deleted. Use the Cap Open boundaries tool (ctrl+o) and play with the jog in the "slope" settings. Voila a new mesh "spiderweb" closed this flat surface and rendering looks great on this. NOTE. do this when you are ready / done with modeling. The more mesh you have the harder you can work with it.