This webpage is using significant energy. Closing it may improve the responsiveness of your Mac? WHAT!

A user cannot do much about this other than what the alert requests.

If the Macbook is on battery, this will have an effect.

Clear all website data

https://support.apple.com/guide/safari/manage-cookies-and-website-data-sfri11471/mac

Enable Develop menu, click Develop in the menu bar  and  then Empty Caches from the dropdown.

https://support.apple.com/guide/safari/use-the-developer-tools-in-the-develop-menu-sfri20948/mac

Clear History: https://support.apple.com/guide/safari/clear-your-browsing-history-sfri47acf5d6/mac

Stop video Auto-play with sound

1. Visit the site a user wants to disable video autoplay.
2. Click Safari in the Safari menu bar and choose “Settings for This Website”

3. Click inside the drop-down box next to  Auto-Play and choose “Never Auto-Play. 

Video should stop playing automatically.

If the user wants to play the video, hit the play button.

Try an Ad blocker Safari Extensions.

Click Safari in the menu bar and choose "Safari Extensions"

Download and install one you like.