検証コード

namespace ClassLibrary1;

public static class MethodExtensions
{
    public static int ExtensionMethod(this object s) => 0;
    public static int ExtensionMethodInt(this int s) => 0;
    public static int ExtensionMethodRefInt(this ref int s) => 0;
}

using ClassLibrary1;

BenchmarkDotNet.Running.BenchmarkRunner.Run<Sample>(new BenchmarkDotNet.Configs.DebugInProcessConfig());

public class Sample
{
    int i = 0;
    [BenchmarkDotNet.Attributes.Benchmark]
    public void MethodObject() =>i.ExtensionMethod();
    [BenchmarkDotNet.Attributes.Benchmark]
    public void MethodInt() => i.ExtensionMethodInt();
    [BenchmarkDotNet.Attributes.Benchmark]
    public void MethodRefInt() => i.ExtensionMethodRefInt();
}