It turns out HashSet<string> is SUPER fast!

gistfile1.txt

using System;
using System.Collections.Generic;
using System.Diagnostics;
using System.Linq;
using System.Text;
using System.Text.RegularExpressions;
using System.Threading.Tasks;

namespace tpl_await_test
{
    class StringTest
    {
        static Random rnd = new Random();
        static string chars = "1234567890qwertyuiopasdfghjklzxcvbnm.";
        static string[] good = GenerateStrings(100).ToArray();
        static string[] all = good.Concat(GenerateStrings(100)).OrderBy(s => rnd.Next()).ToArray();

        private static IEnumerable<string> GenerateStrings(int c)
        {
            for (var i = 0; i < c; i++)
                yield return GenerateString();
        }

        private static string GenerateString()
        {
            var sb = new StringBuilder();
            var l = rnd.Next(15, 35);
            for (var i = 0; i < l; i++)
                sb.Append(chars[rnd.Next(chars.Length)]);
            return sb.ToString();
        }

        public void Run()
        {
            Contains(1);
            HashSet(1);
            RegexSlow(1);
            RegexFast(1);

            var sw = new Stopwatch();
            var runs = 100000;

            sw.Reset();
            sw.Start();
            Contains(runs);
            sw.Stop();
            Console.WriteLine("Contains  {0}", sw.Elapsed);

            sw.Reset();
            sw.Start();
            HashSet(runs);
            sw.Stop();
            Console.WriteLine("HashSet   {0}", sw.Elapsed);

            sw.Reset();
            sw.Start();
            RegexSlow(runs);
            sw.Stop();
            Console.WriteLine("RegexSlow {0}", sw.Elapsed);

            sw.Reset();
            sw.Start();
            RegexFast(runs);
            sw.Stop();
            Console.WriteLine("RegexFast {0}", sw.Elapsed);
        }


        private void Contains(int p)
        {
            for (var i = 0; i < p; i++)
                good.Contains(all[i % all.Length]);
        }

        HashSet<string> set;
        private void HashSet(int p)
        {
            if (set == null)
                set = new HashSet<string>(good, StringComparer.OrdinalIgnoreCase);
            for (var i = 0; i < p; i++)
                set.Contains(all[i % all.Length]);
        }

        Regex reS;
        private void RegexSlow(int p)
        {
            if (reS == null)
                reS = new Regex(string.Join("|", good.Select(s => Regex.Escape(s)).ToArray()), RegexOptions.IgnoreCase);
            for (var i = 0; i < p; i++)
                reS.IsMatch(all[i % all.Length]);
        }

        Regex reF;
        private void RegexFast(int p)
        {
            if (reF == null)
                reF = new Regex(string.Join("|", good.Select(s => Regex.Escape(s)).ToArray()), RegexOptions.IgnoreCase | RegexOptions.Compiled);
            for (var i = 0; i < p; i++)
                reF.IsMatch(all[i % all.Length]);
        }
    }
}