I'm sure it's all a specifications thing, but I will admit that shutter speed shouldn't matter much to the frame rate (as long as you're near the rated shutter speed). 1/250th is 4ms, and 6.3fps is one frame every 158ms. Shooting at 1/125th takes 8ms, so you're probably looking at one shot every 162ms or 6.2fps. 1/60th is about 17ms, so a shot every 171ms is 5.8fps, etc.