not analog ones, because I won't have enough of those
Quotenot analog ones, because I won't have enough of thoseYou can use a 4051 to multiplex more analogue inputs. Reading an analogue signal with a digital pin will take too long for you in this situation.Look at this project for how to use a 4051:-http://www.thebox.myzen.co.uk/Hardware/MIDI_Footsteps.html
A multiplexer can only read a single signal at a time.
But you've only got one A/D anyway, so you haven't lost anything.
You missunderstand, you can't read more than one signal at a time on an arduino, even a mega. There is only one analogue to digital converter and a multiplexer inside the chip. For what you are doing you will not notice any extra delay from an external multiplexer.
Yes if that is what your program tells it to do.
It takes about 100 microseconds (0.0001 s) to read an analog input, so the maximum reading rate is about 10,000 times a second.
Do you know how long it takes the arduino to read a series of 8 analog inputs?
Is that fast enough for you, I think it will be. You can speed it up if you reduce the resolution but there is no need.
If I then keep my finger there and interrupt another beam, that would also be detected (in +-100 microseconds).
No, on average it will be 400 or so microseconds.