针对leetcode上两数之和问题的两种代码：
第一种：

func twoSum(nums []int, target int) []int {
    m := make(map[int]int)
    l := make([]int, 2, 2)
    for firstIndex, firstValue := range nums{
        difference := target - firstValue
        if lastIndex, ok := m[difference]; ok{
            l[0] = firstIndex
            l[1] = lastIndex
            return l
        }
        m[firstValue] = firstIndex
    }
    return nil
}

第二种：

func twoSum(nums []int, target int) []int {
    m := map[int]int{}
    for firstIndex, firstValue := range nums{
        difference := target - firstValue
        if lastIndex, ok := m[difference]; ok{
            return []int{firstIndex, lastIndex}
        }
        m[firstValue] = firstIndex
    }
    return nil
}

提交发现第二种比第一种时间长了3倍左右，第二种方法的切片cap和len应该也是2，为什么会造成耗时增大？