I noticed this as well. By closing browser tabs it appears the LSE watchlist tab was using the CPU, often to 100%. I posted this recently in a thread 'Watchlist is using high percentage of my laptop CPU?'.
It was passed to LSE technical. No reply yet though.
I think that it's due to the number of companies in the watchlists and only when they are updating prices. Something controls this on the page, for example a timer, which runs for a certain time after page load or until the page loses focus. Then the CPU use will drop.