ThreadPool.QueueUserWorkItem() could be very slow

ThreadPool.QueueUserWorkItem() could be very slow if callback being executed as a work item calls Thread.Sleep(). Consider the following example:

1. using System;
2. using System.Threading;
3.  
4. namespace Test
5. {
6.   class Program
7.   {
8.     const int waitTime = 10000;
9.     static void Main(string[] args)
10.     {
11.       for (int i = 0; i < 10; i++ )
12.         ThreadPool.QueueUserWorkItem( TestProc, i);
13.       Console.ReadKey();
14.     }
15.  
16.     static void TestProc(object stateInfo)
17.     {
18.       Console.WriteLine("Item number #{0} at {1}", (int)stateInfo, DateTime.Now);
19.       Thread.Sleep(waitTime);
20.     }
21.   }
22. }

The problem is that thread pool waits around 500 ms before allocating a new thread.
You may solve it by pre-specifying a minimal amount of threads in your application:
ThreadPool.SetMinThreads(20, 200);