C#

Luc Pattyn wrote:

Generic collections offer at least one constructor that takes an initial capacity

List<int> list = new List<int>();
Console.WriteLine(list.Capacity); // 0

list.Add(0);
Console.WriteLine(list.Capacity); // 4

list.AddRange(new int[] { 1, 2, 3, 4 });
Console.WriteLine(list.Capacity); // 8

list.AddRange(new int[] { 5, 6, 7, 8 });
Console.WriteLine(list.Capacity); // 16

list.AddRange(new int[] { 9, 10, 11, 12, 13, 14, 15, 16 });
Console.WriteLine(list.Capacity); // 32

List<int> list = new List<int>(DesiredCapacity);

1. Set the intial capacity of the list as large or larger than you think you'll need it (as others have mentioned, the List class has a constructor for this purpose). This could potentially cut the time in about half.
2. Use structs rather than classes so the memory will be allocated immediately rather than in small chunks over time.
3. Recycle your objects (that is, don't make new objects... save them for use later).

using System.Linq;

...

List<ADUser> list = ws.GetUserList().ToList();

List<ADUsers> list

ws.GetUserList()

List<WindowsFormApplication1.myService.ADUsers> list = ws.GetUserList().ToList();