もしやと思ってやってみたら Binary の方がほんのちょっと(1% くらい)速いの受ける

ConstDecimal.cs

using BenchmarkDotNet.Attributes;
using BenchmarkDotNet.Running;
using System;
using System.Runtime.InteropServices;

public class DecimalConstantBenchmark
{
    const int N = 10000;

    const decimal ConstDecimal = 123456789.123456789m;

    static ReadOnlySpan<byte> bin => new byte[] { 0, 0, 9, 0, 0, 0, 0, 0, 21, 95, 208, 172, 75, 155, 182, 1 };
    static decimal ReadFromBinary => MemoryMarshal.Cast<byte, decimal>(bin)[0];

    [Benchmark]
    public decimal Const()
    {
        var sum = 0m;
        for (int i = 0; i < N; i++) sum += ConstDecimal;
        return sum;
    }

    [Benchmark]
    public decimal Binary()
    {
        var sum = 0m;
        for (int i = 0; i < N; i++) sum += ReadFromBinary;
        return sum;
    }
}

class Program
{
    static void Main()
    {
        BenchmarkRunner.Run<DecimalConstantBenchmark>();
    }
}