macOS just notified me that one tab in Safari was using so much memory it was threatening performance of my laptop. The website in question? https://idioms.thefreedictionary.com/ It was close to 2Gb RAM usage. Bunch of ad-tracking stuff, I assume, as Firefox and its related processes seem to be using less than 1250mb with 6 tabs open, including the site mentioned in 2 tabs. My firefox has uBlock origin installed.

Seemed related to the second paragraph in your comment