I have this issue from Win 8 through my two upgrades to 8.1 and 10.

CPU is extremely high in all browsers (except IE and Edge, didn't test them as don't plan to use them) when:

Download fast (over 2mbps)
Upload fast
Images-heavy sites
Watching TV online

The issue doesn't occur when I download via offline browser, torrent client, download manager or another program.

All my drivers are up-to-date.

Search and Index service is disabled.

Link Scanner in AVG Free is disabled as known to use much CPU during browsing/downloading.