Constant CPU usage

I often borrow a book. Open it in the reader. Put it in full screen.
Then I might switch attention to something else and switch back to other browser pages.
The problem is that the bookreader uses cpu while not shown on screen.

I expected the cpu usage to be 0% when not in use - but the cpu usage is constantly 3%.

I am using an Chrome browser on an Intel Mac.

Version 123.0.6312.107 (Official Build) (x86_64)
Sonoma 14.3.1 (23D60).