Arduino chrony - interrupts not fast enough?

So, I want to build a device which will measure the speed of airsoft BBs (they fly around 100-150 m/s). To do so, I have a tube, in one wall there is an IR LED and on the other side, there are a few photodiodes. As the BB flies through the tube, it crosses the beam of light, casts a shadow on one of the photodiodes and sends a signal to my arduino. There are three photodiodes in series, one end connected to +5V and the other to pin 2 on arduino nano + a pull down resistor. The device works fine when I just throw the BBs through the tube, it registers every single one of them. But when I shoot them at full speed, the device does not register anything. Are the BBs flying just too fast and thus creating too short of a pulse for the arduino to detect? How can I make this thing work?

This is the code:

int x = 0;
void setup() {
  pinMode(2, INPUT);
  Serial.begin(9600);
  attachInterrupt(0, interrupt, FALLING);  
}

void loop() {
}
void interrupt() {    
  x = x+1;
  Serial.println(x);
}

You should not be using serial prints in an ISR.

Yep, get rid of the serial printing, just accumulate the value and print when finished.

Changed it to:

int x = 0;
void setup() {
  pinMode(2, INPUT);
  Serial.begin(9600);
  attachInterrupt(0, interrupt, FALLING);  
}

void loop() {
  Serial.println(x);
  delay(100);
}
void interrupt() {    
  x = x+1;
}

But that does not fix the problem, the device still registers slow BBs no problem, but cannot register the fast ones.

How short is the pulse? (I have already done the arithmetic, but I want you to do it).

The BB is 6mm in diameter, the photodiodes are 3mm, assuming that the BB is travelling at 150 m/s and the area in which it shadowes the diode sufficiently is say 5mm, then the pulse should be 33us long. (the response time of the diode is in units of nanoseconds, so that should not be a problem)

So, tried using my craptacular "osciloscope" and this is what I got. The pulse is pretty long, but the voltage is only droping by litlle over 2 volts, that is not enough, right?

that is not enough, right?

Correct, it has to get down below about 600mV (IIRC) or so.

Something in the chain has a low response time, unless that scope is not up to recording the waveform properly.

There are three photodiodes in series

Why in series? Can you post the circuit?