issues_18465-repro.cs

using System;
using System.Diagnostics;
using System.Threading;

namespace wx_test
{
    class Program
    {
        static string lettersGOOD = "ABCDEFGHIJKLMNOPQRSTUVWXYZ";
        static string lettersBAD = ".-%&?ABCDEFGHIJKLMNOPQRSTUVWXYZ";

        private static void Go(string sample, int iterations)
        {
            for (int i = 0; i < iterations; i++)
            {
                sample.StartsWith("SI");
            }
        }

        static void Main(string[] args)
        {
            var cult = Thread.CurrentThread.CurrentCulture;
            Console.WriteLine($"{cult}; {cult.CompareInfo}");

            Go(lettersGOOD, 1);
            Go(lettersBAD, 1);

            var stopwatch = Stopwatch.StartNew();
            Go(lettersGOOD, 1000000);
            stopwatch.Stop();
            Console.WriteLine($"GOOD: {stopwatch.Elapsed}");

            stopwatch = Stopwatch.StartNew();
            Go(lettersBAD, 1000000);
            stopwatch.Stop();
            Console.WriteLine($"BAD: {stopwatch.Elapsed}");
        }
    }    
}