LinkedIn Webpage Using Significant Energy on Safari

While on LinkedIn, I received a message from Safari (version 11.1) at the top of my browser window: "This webpage is using significant energy. Closing it may improve the responsiveness of your Mac."

I had just sent an invitation, and both the webpage and Safari were sitting idle for about a minute before the message was displayed. I am running macOS Sierra 10.12.6. I've never seen this message before. Does anyone know why the LinkedIn webpage would begin using "significant" energy, especially after being idle for a minute?