When you are using flash, your shutter speed is not necessarily really what you think it is.

A flash, you see, fires at 1/1000th second or faster.

So if the only light falling onto your subject is the flash (an important qualifier), then the shutter speed will effectively be 1/1000th second. Even if the shutter ia actually open for a long time. It’s like opening a bedroom door and flashing the light switch on for a short flash. Then whether you open the door for 1/10th of a second or for a second to do that flash, you will see the same.

If the ambient light also hits your subject, you wil get a mix. This leads to ghosting, like this:

The image was taken at 1/10th second with me moving the camera rapidly.

What all this means to you? That you can often shoot a picture slowly, say at 1/15th of a second, without materially adversely affecting your foreground, flash-lit subject. You’ll get nice light into the background, and the only thing that will really see motion blur is the background.