https://github.com/dotnet/runtime/issues/36989

Program.cs

using System;
using BenchmarkDotNet.Attributes;
using BenchmarkDotNet.Diagnosers;
using BenchmarkDotNet.Running;

public class Program
{
    static void Main() => BenchmarkRunner.Run<Benchmark>();
}

[MemoryDiagnoser]
public unsafe class Benchmark
{
    private Guid guid = Guid.Empty;

    [Benchmark(Baseline = true)]
    public string FastAllocateString()
    {
        const int length = 32;

        var uuidString = Dodo.Primitives.IL.CoreLib.FastAllocateString(length);
        fixed (char* uuidChars = uuidString)
        {
            FormatN(new Span<char>(uuidChars, length));
        }
        return uuidString;
    }

    [Benchmark]
    public string StringCreate()
    {
        return string.Create(32, this, (span, self) => self.FormatN(span));
    }

    private void FormatN(Span<char> span) => guid.TryFormat(span, out _, "N");
}

results.md

BenchmarkDotNet=v0.12.1, OS=Windows 10.0.18362.836 (1903/May2019Update/19H1)
Intel Core i5-2300 CPU 2.80GHz (Sandy Bridge), 1 CPU, 4 logical and 4 physical cores
.NET Core SDK=5.0.100-preview.4.20258.7
  [Host]     : .NET Core 5.0.0 (CoreCLR 5.0.20.25106, CoreFX 5.0.20.25106), X64 RyuJIT
  DefaultJob : .NET Core 5.0.0 (CoreCLR 5.0.20.25106, CoreFX 5.0.20.25106), X64 RyuJIT

Method	Mean	Error	StdDev	Ratio	Gen 0	Gen 1	Gen 2	Allocated
FastAllocateString	77.35 ns	0.460 ns	0.430 ns	1.00	0.0280	-	-	88 B
StringCreate	85.58 ns	0.503 ns	0.470 ns	1.11	0.0280	-	-	88 B