Could be a few things depending on the features exposed on the USB camera and how the USB camera acquires normally. You can experiment quickly with the camera by going to NI-MAX and investigating all the parameters for the camera under Devices and Interfaces. There you can also take a snap and see how long it takes after changing the parameters. When you find parameters that work, you can use those to trigger the camera in your application.
Additionally, running a Snap is an inherently bad way to get high frame rate for more than one image. You'll want to configure a grab to achieve a high frame rate. However, you will still need to configure the camera properly before you can get a good frame rate with a grab, same as a snap.
It might also help to know the camera you are using, but if it's a stock web camera, it might be hard to tell what features it supports.
