#define LED 26
#define SWITCH 25

volatile bool ledState = false;

void IRAM_ATTR isr() {
  ledState = !ledState;
}

void setup() {
  pinMode(LED, OUTPUT);
  pinMode(SWITCH, INPUT_PULLUP);
  attachInterrupt(digitalPinToInterrupt(SWITCH), isr, FALLING);
}

void loop() {
  digitalWrite(LED, ledState);
}