Program.cs

using System;
using System.Linq;
using BenchmarkDotNet.Attributes;
using BenchmarkDotNet.Running;
// ReSharper disable InconsistentNaming

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

    public class StructVsClass
    {
        const int LEN = 1000;
        private static readonly StructP3[] _structs = new StructP3[LEN];
        private static readonly StructP3Ro[] _structsRo = new StructP3Ro[LEN];
        private static readonly ClassP3[] _classes = Enumerable.Range(0, LEN).Select(_ => new ClassP3()).ToArray();


        [Benchmark(Baseline = true)]
        public int Class()
        {
            for (int i = 0; i < LEN; i++)
                _classes[i].X += i;
            return 42;
        }

        [Benchmark]
        public int Structs()
        {
            Span<StructP3> arr = _structs;
            for (int i = 0; i < LEN; i++)
                arr[i].X += i;
            return 42;
        }

        [Benchmark]
        public int StructsRo()
        {
            Span<StructP3Ro> arr = _structsRo;
            for (int i = 0; i < LEN; i++)
            {
                (int x, int y, int z) = arr[i];
                arr[i] = new StructP3Ro(x + i, y, z);
            }
            return 42;
        }

    }

    class ClassP3
    {
        public int X;
        public int Y;
        public int Z;
    }

    struct StructP3
    {
        public int X;
        public int Y;
        public int Z;
    }

    readonly struct StructP3Ro
    {
        public readonly int X;
        public readonly int Y;
        public readonly int Z;

        public StructP3Ro(int x, int y, int z)
        {
            X = x;
            Y = y;
            Z = z;
        }

        public void Deconstruct(out int x, out int y, out int z)
        {
            x = X;
            y = Y;
            z = Z;
        }
    }
}

Results.md

Method	Mean	Error	StdDev	Ratio	RatioSD
Class	747.2 ns	14.93 ns	13.23 ns	1.00	0.00
Structs	572.7 ns	1.25 ns	1.17 ns	0.77	0.01
StructsRo	831.8 ns	4.39 ns	4.11 ns	1.11	0.02