Here are some troubleshooting suggestions for if you are seeing a lag between taking guest photos in Photo Booth Upload (PBU).

1.  Check your computer's CPU Usage. Is something pulling a lot of bandwidth that can be alleviated?

2.  How much space do you have on your computer's hard drive? If there is not a lot of space, your computer may begin to run slow.

3.  Check that your DSLR is on AUTO White Balance in Camera Settings > DSLR Settings.  

4.  Check that all three Camera Mode's are set to DSLR (Or the camera you want them set on).  (Photo Camera, Video Camera, Main Screen Camera.) If one is set to your slow motion video, or your webcam the system has to take an extra moment to switch between cameras between each shot.