Why header image is so heavy?

I'm not sure if you figured this out yet, but you can comment out line 29 of style.css to remove the header image entirely.
Alternately, you could replace header.png with a smaller file. Using GIMP, I saved it as a .jpg at 50% quality. The final image was only 40 kb. You could upload the new image and modify style.css to point to it.

It is fairly difficult to change the image to JPG without it looking very ugly. I have decided to try a GIF image out instead. It doesn't look quite as good as the original PNG image, but it doesn't degrade the image too much, and it's much lighter at 140KB. Please look for this update in the new dev snapshot, and 7.x-2.0.