Just messing around, seeing how to reverse strings faster. Add benchmarkdotnet. Use C# 7.3, Core2.1+. Build Release.

text-reverse-speeds.cs

using System;
using System.Buffers;
using System.Collections.Generic;
using System.Net.Http;
using System.Runtime.CompilerServices;
using System.Runtime.InteropServices;
using System.Text;
using System.Threading;
using BenchmarkDotNet.Attributes;
using BenchmarkDotNet.Running;

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

    [SimpleJob, RankColumn, MemoryDiagnoser]
    public class ReverseTests
    {
        static Lazy<string> SomeLongBitOfText = new Lazy<string>(() =>
            new HttpClient()
                .GetStringAsync(@"https://raw.githubusercontent.com/hunspell/hunspell/6014af96eb5830103261b803ed9b90fc5d360440/tests/suggestiontest/List_of_common_misspellings.txt")
                .GetAwaiter().GetResult());

        private string[] textValues;

        //[Params(1, 100, 1_000_000)]
        public int TestIterations { get; set; } = 100;

        [GlobalSetup]
        public void Setup()
        {
            textValues = SomeLongBitOfText.Value.Split(", \t\r\n".ToCharArray(), StringSplitOptions.RemoveEmptyEntries);
        }

        [Benchmark(Baseline = true)]
        public void ToCharArrayReverse()
        {
            for (var iteration = 0; iteration < TestIterations; iteration++)
            {
                var textValue = textValues[iteration % textValues.Length];

                var result = ToCharArrayReverse(textValue);
            }
        }

        static string ToCharArrayReverse(string textValue)
        {
            var charArray = textValue.ToCharArray();
            Array.Reverse(charArray);
            return new string(charArray);
        }

        [Benchmark]
        public void StringBuilderReverse()
        {
            for (var iteration = 0; iteration < TestIterations; iteration++)
            {
                var textValue = textValues[iteration % textValues.Length];

                var result = StringBuilderReverse(textValue);
            }
        }

        static string StringBuilderReverse(string textValue)
        {
            if (textValue == null || textValue.Length <= 1)
            {
                return textValue;
            }

            var builder = new StringBuilder(textValue);

            var swapOtherIndexOffset = textValue.Length - 1;
            var stopIndex = textValue.Length / 2;
            for (int i = 0, otherIndex = swapOtherIndexOffset; i < stopIndex; i++, otherIndex--)
            {
                var c = builder[i];
                builder[i] = builder[otherIndex];
                builder[otherIndex] = c;
            }

            return builder.ToString();
        }

        [Benchmark]
        public void StringBuilderPoolReverse()
        {
            for (var iteration = 0; iteration < TestIterations; iteration++)
            {
                var textValue = textValues[iteration % textValues.Length];

                var result = StringBuilderPoolReverse(textValue);
            }
        }

        static string StringBuilderPoolReverse(string textValue)
        {
            if (textValue == null || textValue.Length <= 1)
            {
                return textValue;
            }

            var rented = Interlocked.Exchange(ref PooledStringBuilder, null);
            if (rented == null || rented.Capacity < textValue.Length)
            {
                rented = new StringBuilder(textValue.Length);
            }
            else
            {
                rented.Clear();
            }

            for (var i = textValue.Length - 1; i >= 0; i--)
            {
                rented.Append(textValue[i]);
            }

            var result = rented.ToString();
            Interlocked.CompareExchange(ref PooledStringBuilder, rented, null);
            return result;
        }

        [Benchmark]
        public void MemoryPoolReverse()
        {
            for (var iteration = 0; iteration < TestIterations; iteration++)
            {
                var textValue = textValues[iteration % textValues.Length];

                var result = MemoryPoolReverse(textValue);
            }
        }

        static string MemoryPoolReverse(string textValue)
        {
            using (var mo = MemoryPool<char>.Shared.Rent(textValue.Length))
            {
                var buffer = mo.Memory.Span.Slice(0, textValue.Length);
                for (int i = 0, sourceIndex = textValue.Length - 1; i < buffer.Length; i++, sourceIndex--)
                {
                    buffer[i] = textValue[sourceIndex];
                }

                return buffer.ToString();
            }
        }

        [Benchmark]
        public void HomeBakedShittyMemoryPoolReverse()
        {
            for (var iteration = 0; iteration < TestIterations; iteration++)
            {
                var textValue = textValues[iteration % textValues.Length];

                var result = HomeBakedShittyMemoryPoolReverse(textValue);
            }
        }

        static string HomeBakedShittyMemoryPoolReverse(string textValue)
        {
            var rented = Interlocked.Exchange(ref ShittyMemoryPool, null);
            if (rented == null || rented.Length < textValue.Length)
            {
                rented = new char[textValue.Length];
            }

            for (int i = 0, sourceIndex = textValue.Length - 1; i < textValue.Length; i++, sourceIndex--)
            {
                rented[i] = textValue[sourceIndex];
            }

            var result = new string(rented, 0, textValue.Length);
            Interlocked.CompareExchange(ref ShittyMemoryPool, rented, null);
            return result;
        }

        [Benchmark]
        public void HomeBakedShittyMemoryPoolSpanReverse()
        {
            for (var iteration = 0; iteration < TestIterations; iteration++)
            {
                var textValue = textValues[iteration % textValues.Length];

                var result = HomeBakedShittyMemoryPoolSpanReverse(textValue);
            }
        }


        static string HomeBakedShittyMemoryPoolSpanReverse(string textValue)
        {
            var rented = Interlocked.Exchange(ref ShittyMemoryPool, null);
            if (rented == null || rented.Length < textValue.Length)
            {
                rented = new char[textValue.Length];
            }

            var buffer = rented.AsSpan().Slice(0, textValue.Length);
            for (int i = 0, sourceIndex = textValue.Length - 1; i < buffer.Length; i++, sourceIndex--)
            {
                buffer[i] = textValue[sourceIndex];
            }

            var result = buffer.ToString();
            Interlocked.CompareExchange(ref ShittyMemoryPool, rented, null);
            return result;
        }

        [Benchmark]
        public void CreateReverse()
        {
            for (var iteration = 0; iteration < TestIterations; iteration++)
            {
                var textValue = textValues[iteration % textValues.Length];

                var result = CreateReverse(textValue);
            }
        }

        static string CreateReverse(string textValue) =>
            string.Create(textValue.Length, textValue, CreateReverse_DoerThingy);

        static void CreateReverse_DoerThingy(Span<char> buffer, string original)
        {
            for (int index = 0, reverseIndex = buffer.Length - 1; index < buffer.Length; index++, reverseIndex--)
            {
                buffer[index] = original[reverseIndex];
            }
        }

        [Benchmark]
        public void CreateReverseButABitMoreSpanny()
        {
            for (var iteration = 0; iteration < TestIterations; iteration++)
            {
                var textValue = textValues[iteration % textValues.Length];

                var result = CreateReverseButABitMoreSpanny(textValue);
            }
        }

        static string CreateReverseButABitMoreSpanny(string textValue) =>
            string.Create(textValue.Length, textValue.AsMemory(), CreateReverseButABitMoreSpanny_DoerThingy);

        static void CreateReverseButABitMoreSpanny_DoerThingy(Span<char> buffer, ReadOnlyMemory<char> original)
        {
            var originalSpan = original.Span;
            for (int index = 0, reverseIndex = buffer.Length - 1; index < buffer.Length; index++, reverseIndex--)
            {
                buffer[index] = originalSpan[reverseIndex];
            }
        }

        [Benchmark]
        public void UnsafeMutableString()
        {
            for (var iteration = 0; iteration < TestIterations; iteration++)
            {
                var textValue = textValues[iteration % textValues.Length];

                var result = UnsafeMutableString(textValue);
            }
        }

        unsafe static string UnsafeMutableString(string textValue)
        {
            var newTextValue = new string(textValue);
            var indexLimit = newTextValue.Length / 2;
            fixed (char* p = newTextValue)
            {
                for (int i = 0, otherI = newTextValue.Length - 1; i < indexLimit; i++, otherI--)
                {
                    var swap = p[i];
                    p[i] = p[otherI];
                    p[otherI] = swap;
                }
            }
            return newTextValue;
        }

        [Benchmark]
        public void UnsafeMutableSwapString()
        {
            for (var iteration = 0; iteration < TestIterations; iteration++)
            {
                var textValue = textValues[iteration % textValues.Length];

                var result = UnsafeMutableSwapString(textValue);
            }
        }

        unsafe static string UnsafeMutableSwapString(string textValue)
        {
            var newTextValue = new string(textValue);
            var indexLimit = newTextValue.Length / 2;
            fixed (char* p = newTextValue)
            {
                for (int i = 0, otherI = newTextValue.Length - 1; i < indexLimit; i++, otherI--)
                {
                    Swap(ref p[i], ref p[otherI]);
                }
            }
            return newTextValue;
        }

        [Benchmark]
        public void UnsafeMutableSpanString()
        {
            for (var iteration = 0; iteration < TestIterations; iteration++)
            {
                var textValue = textValues[iteration % textValues.Length];

                var result = UnsafeMutableSpanString(textValue);
            }
        }

        unsafe static string UnsafeMutableSpanString(string textValue)
        {
            var newTextValue = new string(textValue);
            fixed (char* p = newTextValue)
            {
                new Span<char>(p, newTextValue.Length).Reverse();
            }
            return newTextValue;
        }

        static char[] ShittyMemoryPool;

        static StringBuilder PooledStringBuilder;

        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        static void Swap(ref char a, ref char b)
        {
            var temp = a;
            a = b;
            b = temp;
        }
    }
}