@Chules There is a large thread somewhere about this performance issue for the new Macs. I thought i remember someone mentioning some type of workaround to get the performance up, but can't remember exactly. Curious if you install an app like Macs Fan Control so you can monitor the temperature. Does the slowdown happen when a certain temp is reached, or is the slowdown a preventative based on the load?
What happen if you have a fan blowing on it or run it in a cold room? Does that keep the temps down and does that affect the render times?