Program.cs

using System;
using System.Collections.Concurrent;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using BenchmarkDotNet;

namespace ConsoleApplication5
{
    public class Program
    {
        ConcurrentDictionary<int, ConcurrentStack<int>> _freeSegments = new ConcurrentDictionary<int, ConcurrentStack<int>>();
        private readonly Func<int, ConcurrentStack<int>> _valueFactory =  i => new ConcurrentStack<int>();
        private static readonly Func<int, ConcurrentStack<int>> _staticValueFactory = i => new ConcurrentStack<int>(); 
        

        static void Main(string[] args)
        {
            new BenchmarkRunner().Run<Program>();

        }

        [Benchmark]
        [OperationsPerInvoke(10)]
        public void CompilerDoesWork()
        {
            for (int i = 0; i < 10; i++)
            {
                var q = _freeSegments.GetOrAdd(2, size => new ConcurrentStack<int>());
                q.Push(3);
                if (q.Count > 1024)
                    q.Clear();
            }
        }

        [Benchmark]
        [OperationsPerInvoke(10)]
        public void ReadOnlyField()
        {
            for (int i = 0; i < 10; i++)
            {
                var q = _freeSegments.GetOrAdd(2, _valueFactory);
                q.Push(3);
                if (q.Count > 1024)
                    q.Clear();
            }
        }

        [Benchmark]
        [OperationsPerInvoke(10)]
        public void ReadOnlyFieldStatic()
        {
            for (int i = 0; i < 10; i++)
            {
                var q = _freeSegments.GetOrAdd(2, _staticValueFactory);
                q.Push(3);
                if (q.Count > 1024)
                    q.Clear();
            }
        }
    }
}

results.txt

BenchmarkDotNet=v0.8.0.0
OS=Microsoft Windows NT 6.2.9200.0
Processor=Intel(R) Core(TM) i7-4710MQ CPU @ 2.50GHz, ProcessorCount=8
HostCLR=MS.NET 4.0.30319.42000, Arch=32-bit
Type=Program  Mode=Throughput  Platform=HostPlatform  Jit=HostJit  .NET=HostFramework  toolchain=Classic  Runtime=Clr  Warmup=5  Target=10
              Method |     AvrTime |    StdDev |         op/s |
-------------------- |------------ |---------- |------------- |
    CompilerDoesWork | 695.5009 ns | 4.1586 ns | 1,437,862.04 |
       ReadOnlyField | 679.8743 ns | 5.9000 ns | 1,470,966.85 |
 ReadOnlyFieldStatic | 686.0820 ns | 7.5863 ns | 1,457,724.95 |


BenchmarkDotNet=v0.8.0.0
OS=Microsoft Windows NT 6.2.9200.0
Processor=Intel(R) Core(TM) i7-4710MQ CPU @ 2.50GHz, ProcessorCount=8
HostCLR=MS.NET 4.0.30319.42000, Arch=64-bit  [RyuJIT]
Type=Program  Mode=Throughput  Platform=HostPlatform  Jit=HostJit  .NET=HostFramework  toolchain=Classic  Runtime=Clr  Warmup=5  Target=10
              Method |     AvrTime |     StdDev |         op/s |
-------------------- |------------ |----------- |------------- |
    CompilerDoesWork | 712.0287 ns |  8.1491 ns | 1,404,617.41 |
       ReadOnlyField | 829.7725 ns | 75.5855 ns | 1,214,984.00 |
 ReadOnlyFieldStatic | 798.9948 ns | 70.2898 ns | 1,260,914.23 |