The most important setting is Properties/Age Profile Settings/ Step
. Higher values place samples further apart, set this as high as you can get away with !
Another important way to improve speed is with the compositing tag. Place a composite tag on the object you are linking in white Lights(e.g a spline) and disable things like reflections or shadows.Disabling WL via the comp tag in a GI scene will speed things up A LOT!