How does tile size affects render time in GPU compute cycles? I do some tests on a scene with various tiles size. The first time, i set the tiles size to 256x256. It rendered in about 9 minutes. The second time, i set it to 512x512.

Now because the tile size is bigger than the first, i thought it would have rendered much faster, but apparently it finished in 12 minutes. How is this so?
You have to see the cores of your graphic card. The number of cores is the number of tiles it can process at the same time. If you have a big amount of cores make the tiles smaller. About 1/8 of the blender's default size is ok. If you have a really good graphic card try the 1/16 of the blender's default size.