After checking the Activity Monitor, I concluded that open tabs consume a significant amount of memory. For instance, the kit.com tab was using 1.2 GB of memory and that usage kept increasing as I used the email editor there for an hour. This was the case for all the tabs.
If you're experiencing similar high memory usage issues, the best solution is to enable Low Power Mode, which should help address the problem.
I appreciate that they did not suspend pinned tabs, allowing important tabs to remain active. After three hours of continuous use, I noticed that memory usage did not exceed 5 GB.