Remember when you check FPS with Leap Motion Visualizer to show your hand, because otherwise controller keeps FPS low.
Frame rate (FPS) depends mainly on USB port/bus and CPU of your PC, from an hardware point of view and on the USB/leap motion controller driver from a software point of view.
So I suggest firstly to plug the Leap Motion Controller on different USB ports on your PC checking FPS and to unplugged other USB devices that can saturate the bus.