Old thread about this here:
Hi, Several times per day, I see this banner in Safari: This webpage is using significant energy. Closing it may improve the responsiveness of your Mac. The website is what it is and there's nothing I can do to improve it. But this is banner happens too often. Is there a way to prevent Safari...
forums.macrumors.com
A couple other old threads on a similar message ("significant memory"):
As per the subject title, webpages in Safari are frequently showing this message. I suspect it is a Catalina issue... I was just wondering if anyone else was experiencing this? Thanks.
forums.macrumors.com
Keep getting "this webpage is using significant memory" along the top of the screen when i watch videos on YT. Any idea how to fix this ? Using the new M1 Macbook. It's happening on even 1080p videos although these videos have no trouble loading/playing. Thank you.
forums.macrumors.com
In Safari on the Mac (but not iPad) you can go to the Safari > Preferences > Advanced tab and check the box to show the developer menu. You can then go to the Develop menu and choose Show Web Inspector. That will show exactly what is going on with the site, but it might be hard to understand if you don't have any experience.
Basically, I think the message means just what it says. There's something about that site that is using a lot of resources which makes your device work harder and use more energy. There could be a lot of reasons for this, it doesn't necessarily mean there's anything wrong with the site.