Will check the laptop in the evening. I'm now at my desktop PC, old Core2 CPU E7300, integrated GPU. Linux. CPU stays at 45%. Initial load maybe up to 60%
On laptop: i5-6267U, 4 cores, Win 10. Loaded the page. Plugged in, CPU runs at 2.9GHz, 35%, fans going into high speed. When on battery, CPU runs at 1.2GHz, 12%, fans not turning...
I have tested it, in chromium. It eats 4 cores fully with almost 100% load. I dont think this has anything to do with the GPU unless you use WebGL in your site.