I am using Nikon D750 camera and Nikon Sb910 flash. I am missing the timing of action because of this delay of flash firing.
Red-eye, a reflection of the flash tinted by the color of the retina, spoils many flash shots. The two short pre-flashes is intended to cause the iris to contract in hopes of mitigating. Setting the camera's aperture for the flash is important. The camera logic will measure the intensity of the reflected light from the pre-flash and use the data to set the correct exposure, it will be different than an exposure based on the ambient illumination level.
Red-eye is likely if the flash is located near the camera lens (on axis). Often professionals use a dismounted flash so its placement is off axis. Additionally, the shadows cast by a dismounted flash creates an illusion of depth.