I'm running Windows 8.1 on an i5 with Chrome and didn't notice any slowdown (nor fan speedup, or any of the usual issues sudden resource drains tend to cause) at all either. Looking at task manager shows Chrome is only using 13% of the CPU while I'm actively viewing that page.

I'm sure YMMV depending upon OS, browser, underlying installed codecs handling the video (and their associated gpu acceleration or lack thereof), etc.