Safari is eating a lot of CPU. When i only have MR opened and it is eating 30% of the CPU. In activity monitor there is an process which is called "Safari Web content" and it is really high.
Disable your Safari extensions, one at a time, and see if one of them is the culprit. I just uninstalled an outdated Safari extension and Voila!! It's like I'm using a brand-new computer. 🙂