Hardware interrupt to ISR execution latency on Photon 2

Hardware
2

I added/changed some code below which might help reduce the time delay. I compiled it but did not run it.
First, I would add only the first line to your code and see if it makes a difference. If not, add the rest of the changes and see what happens. The docs referenced in the code below cautions against using most API functions, with the exception of pinSetFast, pinResetFast, and analogRead, within an ISR.
Hope this helps! :grinning:

SYSTEM_THREAD(ENABLED);// added
#define PIN_OUT   D6
#define PIN_IN   D19

//volatile bool current;
bool current;// changed

volatile bool triggerDetect = false;// added

void isr_in() {// changed
    triggerDetect = true;//Generally, an ISR should be as short and fast as possible.
}

void captureTriggerInfo() {// added
    current = !current;
    digitalWriteFast(PIN_OUT, current);
    digitalWriteFast(PIN_OUT, !current);
    digitalWriteFast(PIN_OUT, current);
}

void setup() {
    pinMode(PIN_OUT, OUTPUT);
    pinMode(PIN_IN, INPUT_PULLDOWN);

    attachInterrupt(PIN_IN, isr_in, CHANGE, 0); // highest priority
    //https://docs.particle.io/reference/device-os/api/interrupts/attachinterrupt/
    
    current = pinReadFast(PIN_IN);
    digitalWriteFast(PIN_OUT, current);
}

void loop() {   
  if (triggerDetect) {
    triggerDetect = false;
    captureTriggerInfo();
  }
}

added:
BTW, there are two other threads with content I thought were interesting and are worth a look:
@rickkas7 Can the P2 run two Interrupts concurrently? - #4 by rickkas7
&
@peekay123 P2 - GPIO and Interrupt Latency