Why request speed grow up when Fiddler opened?

Question

I use my stress test tool, writing in C#

Many tasks run in parallel and standart result ~ 40-50 requests per second. But when i turn on Fiddler - requests grow up to ~ 300-500 and server process it all.

Where is bottleneck in noFiddler variant ?

Simple task

    public class PersonListTask : LTask
    {
        public override string Path => "/person/list?query={page: 1, pageSize: 30}";
        public override string Method => "GET";

        public override async Task ProcessAsync(IUserContext context)
        {
            var request = CreateRequestForContext(context); //WebRequest

            var response = await request.GetResponseAsync();

            if (response == null)
                throw new Exception("Responce null");

            var responceStream = new StreamReader(response.GetResponseStream(), Encoding.UTF8);

            responceStream.ReadToEnd();

            Interlocked.Increment(ref RequestCount);
        }
    }

     ..........
     ..........

     //running 
     private async Task Task_Function(IUserContext context)
     {
            while (true)
            {
                await Task.WhenAll(LTaskCollection.Tasks.Select(x => x.ProcessAsync(context)));
            }
      }