Same here on Firefox 31 on Windows 8.1. 25% used on my dual-core i5 with HT. This just sucks.

Years ago I added a 'snow' animation JS libary to a custom website which let little snow flakes drop during winter time. All just for fun. It turned out to be fun only for a few days until I got the first complaints about users not being able to use their computers anymore and mobile devices being barely able to scroll anything.

I also believe that there must be a better way for simple animations.