UPDATE: I received the parts I needed and finally had the time to setup and test in my shop.
I'm directing this laser beam across the room into a mirror where it reflects back into this photocell. I placed a large diameter piece of heat shrink, about 1" long, around the photocell like a sleeve to cut down ambient light. At this time there is no glass window between the beam and mirror. When the beam is interrupted an LED illuminates.
The reaction time of the photocell is plenty fast enough. The main loop() in my code is running without any delay. I can swing my hand through the beam in a karate chop motion and reaction is fast enough to trip and light the LED. I can flick my finger (like you were flicking a piece of sand off a table) through the beam and it triggers the event.
I do have one concern so far though. Trying to line up that dot of laser beam in the mirror and direct it back to the photocell is tedious. The slightest touch or bump of either mirror or Arduino and the beam is no longer hitting the photocell. Any ideas on how to make that part easier?
I also am going to need some sort of a small mirror on a swivel joint. The mirror I have now is about 2"x3" and no swivel. Something like a motorcycle rear-view mirror would work, but that would be way too big and noticeable on my porch. Any ideas?