Nikon D7100 remote flash?
I just got my SB 700s in the mail and I set them up on commander mode in my camera, but there's a really bad delay before the flash fires. The autofocus light will flash then the flash itself will fire.
The problem is NOT with your flash or camera
The problem is that you are shooting in light that is so low, that the camera has to use the auto-focus assist lamp to assure good focus.
All this takes time, so there will be a delay between the time you press the shutter release, the lens focuses and the shutter actually operates.
If you can't boost the ambient lighting, you can have someone aim an small LED light at the subject and then your camera will focus easily and the flash will not only fire right away, but it will also overcome the light projected by the LED light. I have also had success if I'm attempting to photograph a person in very low light by having them light up the screen on their mobile (cell phone) and hold it at about chest level.
That's the way the camera and flash was designed to work.
When there's insufficient light to focus, the focus assist light will come on. Your lens/camera will acquire focus and lock. THEN, the master flash will trigger the remote flash.
Yes, there will be a delay between focus assist light and the flash. Darker it is, longer it takes for your camera to acquire focus, also.
Since your lens/camera takes a moment to acquire focus and lock the focus in any case, there has to be that delay. The shutter and the flash should be in sync. That's the only important thing.
I use a SB700 and SB900 with my D7100 and have no such issues.
Double check your settings, make sure you are not set on rear-curtain sync, red-eye, modelling flash, and so on.
How long is "really bad delay"?
In low light my Canon the first shot takes maybe a second or 2 to achieve focus depending on lens, ambient light and distance to subject. Assuming no changes then immediately subsequent shots focus almost instantly.
If yours is taking any longer than that then you may have a faulty unit.