generate_page_header_parallax_speed

The generate_page_header_parallax_speed filter allows you to adjust the speed at which the page header parallax script moves your background image.
The default speed is 2, and I wouldn』t suggest going much lower than 1.5. The level of effect is dependent on the height of your image, as well.
Usage
add_filter( 'generate_page_header_parallax_speed', 'tu_adjust_page_header_parallax' );
function tu_adjust_page_header_parallax() {
return 2;
}