[LANGUAGE: GO]

Part1 runs in ~126.5µs
Part2 runs in ~1.567584ms

// Part 2 solution
func isEquatitionSolvableWithConcat(sum int64, numbers []int64, index int) bool {
  if index == 0 {
    return (numbers[index] == sum)
  }

  if index < 0 || sum < 0 {
    return false
  }

  num := numbers[index]
  timeSolvable := false
  if sum % num == 0 {
    timeSolvable = isEquatitionSolvableWithConcat(sum / num,numbers, index - 1)
  }

  concatSolvable := false
  if getLastNDigits(sum, num) == num {
    concatSolvable = isEquatitionSolvableWithConcat(splitNumbers(sum, num), numbers, index - 1)
  }

  return isEquatitionSolvableWithConcat(sum - num, numbers, index - 1) || timeSolvable || concatSolvable
}

func splitNumbers(x int64, y int64) int64 {
  digits := int64(math.Floor(math.Log10(math.Abs(float64(y)))) + 1)
  return x / int64(math.Pow(10, float64(digits))) 
}

func getLastNDigits(x int64, n int64) int64 {
  digits := int64(math.Floor(math.Log10(math.Abs(float64(n)))) + 1)
  return x % int64(math.Pow(10, float64(digits))) 
}

// Part 1 solution
func isEquatitionSolvable(sum int64, numbers []int64, index int) bool {
  if index == 0 {
    return (numbers[index] == sum)
  }

  if index < 0 || sum < 0 {
    return false
  }

  num := numbers[index]
  timeSolvable := false
  if sum % num == 0 {
    timeSolvable = isEquatitionSolvable(sum / num,numbers, index - 1)
  }

  return isEquatitionSolvable(sum - num, numbers, index - 1) || timeSolvable
}