Hoping the user doesn't notice that you've been using their resources has always seemed like an awful gamble to me: both in the sense that you're deceiving your users by pinning the blame on other websites/their browser/their computer in the their mind, and also in the sense that you're just making it more likely that someday users will have access to this information.
It's pretty noticeable which websites hog resources if you use your laptop or phone regularly. People are well aware that streaming and social media apps sinks battery like a boulder compared to looking at a wikipedia article. My macbook is dead in 5 hours or 1 hour depending on if I'm on reddit/hn or content heavy websites. Fans kick into gear when netflix starts streaming and the laptop becomes untouchable. Back when I used messenger on my phone I used to watch my battery percent tick down by the minute.