Many cameras require that their focus input is asserted before they will react to a shutter signal. This depends on make, model and camera settings. If possible, it is advised to set the camera to full manual exposure mode and disable autofocus to achieve the most consistent timings.
Many common camera model performance values can be found at http://www.imaging-resource.com/cameras/reviews/. While these values may not be definitive for every use case, they are a good starting point.