What's Causing High CPU Use On GMail?

I've seen this on Opera, Vivaldi and Firefox. When a open a gmail page the cpu goes up and keeps there around 2min/3min. If I open my two email pages then it goes crazy... some time ago I left as fixed tab on opera both gmail pages and the opening of the browser was a pain! I'm using laptop, that's because I'm worried/curious about this, If someone has any ideia...