ListPerformance.kt

val itemsToAdd = 500_000

// val list = mutableListOf<Int>()
val tMutableList = measureNanoTime {
    val list = mutableListOf<Int>()
    for (i in 1..itemsToAdd) {
        list += i
    }
}
Timber.i("Using += on `MutableList<Int>` took ${TimeUnit.NANOSECONDS.toMillis(tMutableList)}ms")


// var list: List<Int> = mutableListOf()
val t = measureNanoTime {
    var list: List<Int> = mutableListOf()
    for (i in 1..itemsToAdd) {
       list += i
    }
}
Timber.i("Using += on `List<Int>` took ${TimeUnit.NANOSECONDS.toMillis(t)}ms")

Thanks for the comment.

I've since updated the post to highlight a further nuance: that += will have different behaviour depending on how you define the list. If the list is declared as a List<Int>, then += will copy the whole list and have terrible performance. but if the list is declared as a MutableList<Int>, or left not explicitly declared when assigning to ArrayList<Int> (as in your example), then += will use the add() method directly and be performant.