_Results.md

BenchmarkDotNet=v0.12.1, OS=Windows 10.0.18363.778 (1909/November2018Update/19H2)
Intel Core i7-8850H CPU 2.60GHz (Coffee Lake), 1 CPU, 12 logical and 6 physical cores
.NET Core SDK=3.1.300-preview-015048
  [Host]     : .NET Core 3.1.3 (CoreCLR 4.700.20.11803, CoreFX 4.700.20.12001), X64 RyuJIT
  DefaultJob : .NET Core 3.1.3 (CoreCLR 4.700.20.11803, CoreFX 4.700.20.12001), X64 RyuJIT

Method	Mean	Error	StdDev	Median	Gen 0	Gen 1	Gen 2	Allocated
FromString	315.142 μs	5.8171 μs	10.6369 μs	315.401 μs	4.3945	-	-	17.92 KB
FromBytes	261.120 μs	4.9478 μs	6.7726 μs	260.883 μs	0.9766	-	-	5.63 KB
FromStream	261.504 μs	6.6877 μs	18.8627 μs	253.325 μs	0.9766	-	-	4.58 KB
ToEncodedString	6.436 μs	0.1246 μs	0.1576 μs	6.428 μs	0.7782	-	-	3.59 KB
ToMemoryStream	5.925 μs	0.1172 μs	0.2692 μs	5.846 μs	0.3738	-	-	1.73 KB

ConsoleApp87.csproj

<Project Sdk="Microsoft.NET.Sdk">

  <PropertyGroup>
    <OutputType>Exe</OutputType>
    <TargetFramework>netcoreapp3.1</TargetFramework>
  </PropertyGroup>

  <ItemGroup>
    <PackageReference Include="BenchmarkDotNet" Version="0.12.1" />
  </ItemGroup>

  <ItemGroup>
    <None Update="Test.json">
      <CopyToOutputDirectory>PreserveNewest</CopyToOutputDirectory>
    </None>
  </ItemGroup>

</Project>

Program.cs

using BenchmarkDotNet.Attributes;
using BenchmarkDotNet.Running;
using System;
using System.Collections.Generic;
using System.IO;
using System.Text;
using System.Text.Json;
using System.Threading.Tasks;

namespace ConsoleApp87
{
    class Program
    {
        static void Main(string[] args)
        {
            BenchmarkRunner.Run<Tests>();
        }
    }

    [MemoryDiagnoser]
    public class Tests
    {
        static readonly string _asString = @"
[
  {
    ""_id"": ""5ea9fa48a46f46a9c2b2338e"",
    ""index"": 0,
    ""guid"": ""2628b8ef-aaf4-4559-95c4-72d0abe7ab16"",
    ""isActive"": false,
    ""balance"": ""$3,061.25"",
    ""picture"": ""http://placehold.it/32x32"",
    ""age"": 28,
    ""eyeColor"": ""green"",
    ""name"": {
      ""first"": ""Stefanie"",
      ""last"": ""Bryan""
    },
    ""company"": ""LYRICHORD"",
    ""email"": ""stefanie.bryan@lyrichord.org"",
    ""phone"": ""+1 (953) 448-2960"",
    ""address"": ""140 Bleecker Street, Connerton, American Samoa, 1242"",
    ""about"": ""Qui mollit officia velit dolore aute irure minim mollit. Elit consequat cupidatat adipisicing aliquip tempor anim officia aliquip eu non duis dolor. Aliquip do fugiat veniam labore esse. Non ipsum est reprehenderit ad dolor sunt cupidatat quis et."",
    ""registered"": ""Monday, December 25, 2017 1:22 PM"",
    ""latitude"": ""-24.037376"",
    ""longitude"": ""58.896979"",
    ""tags"": [
      ""amet"",
      ""ea"",
      ""quis"",
      ""ea"",
      ""in""
    ],
    ""range"": [
      0,
      1,
      2,
      3,
      4,
      5,
      6,
      7,
      8,
      9
    ],
    ""friends"": [
      {
        ""id"": 0,
        ""name"": ""Camille Langley""
      },
      {
        ""id"": 1,
        ""name"": ""Rhodes Combs""
      },
      {
        ""id"": 2,
        ""name"": ""Reilly Bryant""
      }
    ],
    ""greeting"": ""Hello, Stefanie! You have 6 unread messages."",
    ""favoriteFruit"": ""apple""
  }
]
";

        static readonly Example[] _asType = JsonSerializer.Deserialize<Example[]>(_asString);

        [Benchmark]
        public async Task<Example[]> FromString()
        {
            var json = await File.ReadAllTextAsync("Test.json");
            return JsonSerializer.Deserialize<Example[]>(json);
        }

        [Benchmark]
        public async Task<Example[]> FromBytes()
        {
            var json = await File.ReadAllBytesAsync("Test.json");
            return JsonSerializer.Deserialize<Example[]>(json);
        }

        [Benchmark]
        public async Task<Example[]> FromStream()
        {
            using var json = File.OpenRead("Test.json");
            return await JsonSerializer.DeserializeAsync<Example[]>(json);
        }

        [Benchmark]
        public byte[] ToEncodedString()
        {
            var json = JsonSerializer.Serialize(_asType);
            return Encoding.UTF8.GetBytes(json);
        }

        [Benchmark]
        public async Task<MemoryStream> ToMemoryStream()
        {
            var memory = new MemoryStream();
            
            await JsonSerializer.SerializeAsync(memory, _asType);

            memory.Position = 0;

            return memory;
        }
    }

    public class Name
    {
        public string first { get; set; }
        public string last { get; set; }
    }

    public class Friend
    {
        public int id { get; set; }
        public string name { get; set; }
    }

    public class Example
    {
        public string _id { get; set; }
        public int index { get; set; }
        public string guid { get; set; }
        public bool isActive { get; set; }
        public string balance { get; set; }
        public string picture { get; set; }
        public int age { get; set; }
        public string eyeColor { get; set; }
        public Name name { get; set; }
        public string company { get; set; }
        public string email { get; set; }
        public string phone { get; set; }
        public string address { get; set; }
        public string about { get; set; }
        public string registered { get; set; }
        public string latitude { get; set; }
        public string longitude { get; set; }
        public List<string> tags { get; set; }
        public List<int> range { get; set; }
        public List<Friend> friends { get; set; }
        public string greeting { get; set; }
        public string favoriteFruit { get; set; }
    }
}

Test.json

[
  {
    "_id": "5ea9fa48a46f46a9c2b2338e",
    "index": 0,
    "guid": "2628b8ef-aaf4-4559-95c4-72d0abe7ab16",
    "isActive": false,
    "balance": "$3,061.25",
    "picture": "http://placehold.it/32x32",
    "age": 28,
    "eyeColor": "green",
    "name": {
      "first": "Stefanie",
      "last": "Bryan"
    },
    "company": "LYRICHORD",
    "email": "stefanie.bryan@lyrichord.org",
    "phone": "+1 (953) 448-2960",
    "address": "140 Bleecker Street, Connerton, American Samoa, 1242",
    "about": "Qui mollit officia velit dolore aute irure minim mollit. Elit consequat cupidatat adipisicing aliquip tempor anim officia aliquip eu non duis dolor. Aliquip do fugiat veniam labore esse. Non ipsum est reprehenderit ad dolor sunt cupidatat quis et.",
    "registered": "Monday, December 25, 2017 1:22 PM",
    "latitude": "-24.037376",
    "longitude": "58.896979",
    "tags": [
      "amet",
      "ea",
      "quis",
      "ea",
      "in"
    ],
    "range": [
      0,
      1,
      2,
      3,
      4,
      5,
      6,
      7,
      8,
      9
    ],
    "friends": [
      {
        "id": 0,
        "name": "Camille Langley"
      },
      {
        "id": 1,
        "name": "Rhodes Combs"
      },
      {
        "id": 2,
        "name": "Reilly Bryant"
      }
    ],
    "greeting": "Hello, Stefanie! You have 6 unread messages.",
    "favoriteFruit": "apple"
  }
]