Performance discussion from https://www.reddit.com/r/csharp/comments/12bgm2x/why_is_listexists_slower_in_c_than_find_in_js/

Program.cs

using System.Diagnostics;
using System.Security.Cryptography;
using BenchmarkDotNet.Attributes;
using BenchmarkDotNet.Running;

namespace CsTest;

[SimpleJob]
public class MainClass
{
    public static void Main( string[] args )
    {
        // var summary = BenchmarkRunner.Run<MainClass>();
        var cMainClass = new MainClass();
        var setupWatch = Stopwatch.StartNew();
        cMainClass.Setup();
        Console.WriteLine( $"Setup: {setupWatch.ElapsedMilliseconds}ms" );
        var watch = Stopwatch.StartNew();
        cMainClass.Test();
        watch.Stop();
        Console.WriteLine( $"Test: {watch.ElapsedMilliseconds}ms" );
        Console.WriteLine( $"Items Found: {cMainClass._list.Count}" );

        var watch2 = Stopwatch.StartNew();
        cMainClass.Test2();
        watch2.Stop();
        Console.WriteLine( $"Test with dict: {watch2.ElapsedMilliseconds}ms" );
        Console.WriteLine( $"Items Found: {cMainClass._list.Count}" );
    }

    private readonly List<Obj> _list = new();
    private readonly Dictionary<int, Obj> _dict = new();
    private readonly List<int> _testList = new();

    [GlobalSetup]
    public void Setup()
    {
        for( var i = 0; i < 10000; i++ )
            _list.Add( new Obj
            {
                Id = RandomNumberGenerator.GetInt32( 1, 10000 ),
                Name = "Name",
                Description = "Description",
            } );

        _testList.AddRange( Enumerable.Range( 0, 1000000 ).Select( x => RandomNumberGenerator.GetInt32( 1, 10000 ) ));
    }

    [Benchmark]
    public void Test()
    {
        foreach( var id in _testList )
        { 
            if( _list.Exists( obj => obj.Id == id ) )
                continue;
            _list.Add( new Obj
            {
                Id = id,
                Name = "Name",
                Description = "Description",
            } );
        }
    }

    public void Test2()
    {
        foreach( var id in _testList )
        {
            if( _dict.ContainsKey( id ) )
                continue;
            _dict[id] = new Obj
            {
                Id = id,
                Name = "Name",
                Description = "Description",
            };
        }
    }
}

public class Obj
{
    public int Id { get; set; }
    public string Name { get; set; } = null!;
    public string Description { get; set; } = null!;
}