My PC is a 6 year old rig with Intel core 2 quad. I have 16Gb of RAM and a GTX 660 Ti though, but never the less, it's pretty week by today's standards.
On Chrome I get 60 fps most of the time and the CPU gets as high as 74%. This is on it's peek, with more than 1300+ objects at once.
On Firefox I get 20 fps and 15% CPU, but the fps are not correct because Firefox freezes for 8-12 and performs like a slideshow.
I haven't tested Firefox before, I do all my tests on Chrome because it renders more stable and predicable results and since I am only interested on desktop stuff (which in theory should have the same performance with node webkit). It's obvious to me that Firefox does not utilize the CPU correctly. I would have never thought that it would be so bad though.
istavang, you can also reduce the amount of shadowing objects that each cloud holds. For example, use 12 image points for each cloud frame and increase the size of the shadow object from 8x8 to 16x16 to balance the difference and roughly match the cloud's outline. Finally, reduce the Repeat loop from 24 to 12 times. That should give similar results with half the object count.