ArrayRangePerformance.csproj

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

  <PropertyGroup>
    <OutputType>Exe</OutputType>
    <TargetFramework>netcoreapp3.0</TargetFramework>
  </PropertyGroup>

  <ItemGroup>
    <PackageReference Include="BenchmarkDotNet" Version="0.12.0" />
  </ItemGroup>

</Project>

ArraySum.cs

using System;
using System.Linq;
using BenchmarkDotNet.Attributes;

namespace ArrayRangePerformance
{
	[MemoryDiagnoser]
	public class ArraySum
	{
		private const int Count = 100000000;
		public readonly long[] _array = Enumerable.Range(0, Count).Select(x => (long)x).ToArray();
		public readonly int _start = 99;
		public readonly int _stop = Count - 1;

		[Benchmark]
		public long Sum_Index()
		{
			var sum = 0L;
			for (var i = _start; i < _stop; i++)
			{
				sum += _array[i];
			}

			return sum;
		}

		[Benchmark]
		public long Sum_Linq()
		{
			return _array.Skip(_start).Take(_stop - _start).Sum();
		}

		[Benchmark]
		public long Sum_Range()
		{
			return _array[_start.._stop].Sum();
		}

		[Benchmark]
		public long Sum_Span()
		{
			var sum = 0L;
			foreach (var n in _array.AsSpan(_start.._stop))
			{
				sum += n;
			}

			return sum;
		}
	}
}

Program.cs

using BenchmarkDotNet.Running;

namespace ArrayRangePerformance
{
	class Program
	{
		static void Main()
		{
			BenchmarkRunner.Run<ArraySum>();
		}
	}
}