I noticed a similar difference on my MacBook Pro: 25 s with SunFlow rendering engine vs 70 s with YafaRay engine. Maybe it's because of some compilation options of YafaRay, but it's still weird because it depends on the rendered scene.

Enko, the shadow of transparent objects seem to be more accurate with Yafaray, but I wonder what could make the object color change... Could you share your simple scene, please?