Counting elements in an array -Performance-

Let's carry out horse races in a bit different manner:

private static string UnderTest(int size) {
  int pos = 0, neg = 0, zeros = 0;
  int p = 0, n = 0, z = 0;

  Random random = new Random(0);
  int[] arr = Enumerable
    .Range(0, size)
    .Select(x => random.Next(-1, 2))
    .ToArray();
  GC.Collect(GC.MaxGeneration);

  Stopwatch sw = new Stopwatch();
  sw.Start();

  // Three Linq loops (expected to be 3 three times slower)
  pos = arr.Sum(e => e > 0 ? 1 : 0);    
  neg = arr.Sum(e => e < 0 ? 1 : 0);
  zeros = arr.Sum(e => e == 0 ? 1 : 0);

  sw.Stop();

  Stopwatch sw2 = new Stopwatch();
  sw2.Start();

  // Just 1 loop 
  foreach (var item in arr) {
    if (item > 0) p++;
    else if (item < 0) n++;
    else z++;
  }

  sw2.Stop();

  return $"{sw.Elapsed} vs. {sw2.Elapsed} ratio: {((double)sw.Elapsed.Ticks) / sw2.Elapsed.Ticks:f3}";
} 

Races for different array sizes:

   int[] loops = new int[] {
    1000,
    10_000,
    100_000,
    1_000_000,
    10_000_000,
    100_000_000,
    1000,         // <- 1000 again
  };

  string report = string.Join(Environment.NewLine, loops
    .Select(loop => $"loops: {loop,10} : {UnderTest(loop)}"));

  Console.Write(report);

Outcome:

loops:       1000 : 00:00:00.0006471 vs. 00:00:00.0000114 ratio: 56.763 // <- Warming up
loops:      10000 : 00:00:00.0003195 vs. 00:00:00.0001074 ratio: 2.975
loops:     100000 : 00:00:00.0037131 vs. 00:00:00.0010910 ratio: 3.403
loops:    1000000 : 00:00:00.0351574 vs. 00:00:00.0118858 ratio: 2.958
loops:   10000000 : 00:00:00.3729617 vs. 00:00:00.1198276 ratio: 3.112
loops:  100000000 : 00:00:03.7002508 vs. 00:00:01.1808595 ratio: 3.134
loops:       1000 : 00:00:00.0000322 vs. 00:00:00.0000099 ratio: 3.253 // <- Expected

What's going on: we have 3 Linq based loops (3 calls for Sum), so Linq 3 times slower ( no surprise). However we have a huge outlier at the very first run when system loads assembly, compiles IL code etc. So you have a so called warming up effect.