StackOverflow 36940868, C#

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//Rextester.Program.Main is the entry point for your code. Don't change it.
//Compiler version 4.0.30319.17929 for Microsoft (R) .NET Framework 4.5

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text.RegularExpressions;
using System.Threading;
using System.Diagnostics;

namespace Rextester
{
    class MyClass
    {
        public string WordId { get; set; }
    }
    
    public class Program
    {
        public static void Main(string[] args)
        {

var db = new List<MyClass>();

for (int i = 0; i < 8000; i++) 
            {
                db.Add(new MyClass() { WordId = Guid.NewGuid().ToString() });
            }

Console.WriteLine("total items: " + db.Count);
            
            
            var s = new Stopwatch();

s.Start();

var lettersList = new List<char>() { 'a', 'b', 'c', 'd', 'e', 'A', 'B', 'C', 'D', 'E' };

var result = db.Where(x => lettersList.Contains(x.WordId.First()));                
                
            s.Stop();
            
            Console.WriteLine("\nResult number : " + result.Count());

Console.WriteLine("\nTime elapsed: " + s.ElapsedTicks);
        }
    }
}

Show compiler warnings [ + ] Show input
Compilation time: 0,2 sec, absolute running time: 0,14 sec, cpu time: 0,12 sec, average memory usage: 14 Mb, average nr of threads: 3