for (int i = 0; i < NUM_LEDS; i++) {

Just as an experiment, try this: change this line

for (int i = 0; i < NUM_LEDS + NUM_LEDS; i++) {

to this (remove "+ NUM_LEDS")

for (int i = 0; i < NUM_LEDS; i++) {

This may speed up the "finishing". Downside is that some of the last pieces of the trail do not disappear all that elegantly.
So you could look at fine tuning this to maybe something like this:

for (int i = 0; i < NUM_LEDS + meteorSize; i++) { 

Give it a try and see what happens 😊 

Thanks again.

Welcome to the forum 😉 

First thing; to make things faster, I'd recommend switching to the FastLED library. It's more mature, offers more functions and ... is just faster than NeoPixel.
Because of the better functions, it can make your code a lot shorter as well!

I wrote my project at the time with both NeoPixel and FastLED in mind, and for that I had to write a few small "generic" functions.
FadeToBlack for example is much faster and easier to use in FastLED.

For example, remove the functions fadeToBlack, setPixelColor, showStrip and setPixel and replace all the calls to these functions like so:

fadeToBlack(LedNo, FadeValue) -> leds.fadeToBlackBy( fadeValue );

showStrip() -> FastLED.show();

setPixel(Pixel, red, green, blue) -> leds = CRGB(red, green, blue);

setAll(red, green, blue) -> fill_solid( leds, NUM_LEDS, CRGB( red, green, blue) );

Next, your delay maybe caused by several things, but first thing I'd look at is the meteorTrailDecay value.
It may take more time than expected when the LEDs are almost dark.

#include

#define PIN 2
#define NUM_LEDS 60
// Parameter 1 = number of pixels in strip
// Parameter 2 = pin number (most are valid)
// Parameter 3 = pixel type flags, add together as needed:
// NEO_KHZ800 800 KHz bitstream (most NeoPixel products w/WS2812 LEDs)
// NEO_KHZ400 400 KHz (classic 'v1' (not v2) FLORA pixels, WS2811 drivers)
// NEO_GRB Pixels are wired for GRB bitstream (most NeoPixel products)
// NEO_RGB Pixels are wired for RGB bitstream (v1 FLORA pixels, not v2)
Adafruit_NeoPixel strip = Adafruit_NeoPixel(NUM_LEDS, PIN, NEO_GRB + NEO_KHZ800);

void setup() {
  strip.begin();
  strip.show(); // Initialize all pixels to 'off'
}
// ---------------------------------------------------
void loop() {
  meteorRain(0xff, 0x00, 0x00, 4, 22, true, 8);
}

void meteorRain(byte red, byte green, byte blue, byte meteorSize, byte meteorTrailDecay, boolean meteorRandomDecay, int SpeedDelay) {
  ;

  for (int i = 0; i < NUM_LEDS + NUM_LEDS; i++) {

    // fade brightness all LEDs one step
    for (int j = 0; j < NUM_LEDS; j++) {
      if ((!meteorRandomDecay) || (random(10) > 5)) {
        fadeToBlack(j, meteorTrailDecay);
      }
    }

    // draw meteor
    for (int j = 0; j < meteorSize; j++) {
      if ((i - j < NUM_LEDS) && (i - j >= 0)) {
        setPixel(i - j, red, green, blue);
      }
    }

    showStrip();
    delay(SpeedDelay);
  }
}

void fadeToBlack(int ledNo, byte fadeValue) {
  #ifdef ADAFRUIT_NEOPIXEL_H
  // NeoPixel
  uint32_t oldColor;
  uint8_t r, g, b;
  int value;

  oldColor = strip.getPixelColor(ledNo);
  r = (oldColor & 0x00ff0000 UL) >> 16;
  g = (oldColor & 0x0000ff00 UL) >> 8;
  b = (oldColor & 0x000000ff UL);

  r = (r <= 10) ? 0 : (int) r - (r * fadeValue / 250);
  g = (g <= 10) ? 0 : (int) g - (g * fadeValue / 250);
  b = (b <= 10) ? 0 : (int) b - (b * fadeValue / 250);

  strip.setPixelColor(ledNo, r, g, b);
  #endif
  #ifndef ADAFRUIT_NEOPIXEL_H
  // FastLED
  leds.fadeToBlackBy(fadeValue);
  #endif
}
// -----------------------------------------
void showStrip() {
  #ifdef ADAFRUIT_NEOPIXEL_H
  // NeoPixel
  strip.show();
  #endif
  #ifndef ADAFRUIT_NEOPIXEL_H
  // FastLED
  FastLED.show();
  #endif
}

void setPixel(int Pixel, byte red, byte green, byte blue) {
  #ifdef ADAFRUIT_NEOPIXEL_H
  // NeoPixel
  strip.setPixelColor(Pixel, strip.Color(red, green, blue));
  #endif
  #ifndef ADAFRUIT_NEOPIXEL_H
  // FastLED
  leds.r = red;
  leds.g = green;
  leds.b = blue;
  #endif
}

void setAll(byte red, byte green, byte blue) {
  for (int i = 0; i < NUM_LEDS; i++) {
    setPixel(i, red, green, blue);
  }
  showStrip();
}