It's a final gathering problem. You need to increase the FG precision or switch to a different model. You can get a speed up by going from IBL to final gather mode for indirect lighting. It also may (only) need you to increase IBL shadow precision from 0.5 to a higher number, without actually increasing final gather slider precision.
Most of your problems will be gone by simply switching over to ART. It's mostly compatible with MR. To get the same look (black background with whitish color for a skylight) change background image to "environment background switcher" that you'll set up so that black is the background color and the environment color is your skylight color. Actual skylight light is no longer needed nor supported in ART. Your environment color acts as a skylight of the same color. The only way to get background of one color and light/reflection of another is to use the aforementioned env/bg switcher.
Bear in mind that you need MR installed for env/bg switcher to be available for use in ART. Set ART to 25-33 db with noise filtering on at default 50% before getting a feeling for this extremely simple, photometric rendering engine.
