What is an ideal setup for detecting objects moving through an enclosed space?
You can try a Photo Interrupter.  Such self contained emitter / detector devices are common and can be found in a variety of configurations.  This one has a 10mm opening (ball is 6mm wide) and a max low to high response of 9us + a max high to low of 15us.  Assuming this represents the shortest (worst case) possible detectable obstruction of 24us.  Which, if correct, is &lt;&lt; 70us.

https://www.sparkfun.com/datasheets/Components/GP1A57HRJ00F.pdf