Click the image for a larger view.

Perhaps try these render settings. This is what I use.

The bucket size is variable and what you set it at will largely depend on your computer's memory. It  can be adjusted by values of 256, 128, 64, 32, 16, 8  if you need to lower  the value if you find that you are running out of memory when rendering.  If you have a powerful computer with lots and lots of memory, you can increase the bucket size to 512.

You can save the settings by clicking "Save Preset".  Then when you want to use it again, just pick "load preset" and it will show in the drop down menu.

"It is good to see ourselves as others see us. Try as we may, we are never
able to know ourselves fully as we are, especially the evil side of us.
This we can do only if we are not angry with our critics but will take in good
heart whatever they might have to say." - Ghandi