You are using web client incorrectly. Try
public static void Main(string[] args) {
object lockConsole = new object();
WebClient client = new WebClient();
Stopwatch stopwatch = new Stopwatch();
stopwatch.Start();
Parallel.For(0, 1000, new ParallelOptions { MaxDegreeOfParallelism = 20 },
i => {
try {
string url = $"https://www.merehelene.com/i/test-{i}.jpg?size=50";
Stopwatch sw = new Stopwatch();
sw.Start();
client.DownloadData(url);
lock (lockConsole) {
Console.CursorLeft = 0;
Console.Write($"{sw.ElapsedMilliseconds}ms");
}
} catch (Exception e) {
Console.WriteLine(e.Message);
}
});
Console.WriteLine($"Total time: {stopwatch.ElapsedMilliseconds}ms");
Console.ReadLine();
}