CountTruesBenchmark.cs

   public class CountTruesBenchmarks
    {
        private readonly bool[] boolArray;

        public CountTruesBenchmarks()
        {
            var rnd = new Random(42);
            boolArray = Enumerable.Range(0, 500).Select(_ => rnd.NextBoolean()).ToArray();
        }

        [Benchmark]
        public int CountTruesFor()
        {
            var count = 0;
            for (int i = 0; i < boolArray.Length; i++)
            {
                if (boolArray[i])
                {
                    count++;
                }
            }

            return count;
        }

        [Benchmark]
        public int CountTruesForEach()
        {
            var count = 0;
            foreach (var b in boolArray)
            {
                if (b)
                {
                    count++;
                }
            }

            return count;
        }


        [Benchmark]
        public int CountTruesLinq()
        {
            return boolArray.Count(b => b);
        }

        [Benchmark]
        public unsafe int CountTruesAvx2()
        {
            var count = 0;

            if (Avx2.IsSupported)
            {
                var maxBatchIndex = boolArray.Length - boolArray.Length % Vector256<byte>.Count;
                if (maxBatchIndex > 0)
                {
                    var zerosVector = Vector256<byte>.Zero;
                    fixed (bool* boolArrayPtr = boolArray)
                    {
                        var boolArrayBytePtr = (byte*) boolArrayPtr;
                        for (int i = 0; i < maxBatchIndex; i += Vector256<byte>.Count)
                        {
                            var batchVector = Avx.LoadVector256(boolArrayBytePtr + i);
                            var comparisonResult = Avx2.CompareEqual(batchVector, zerosVector);
                            var nonZeroMask = ~(uint) Avx2.MoveMask(comparisonResult);
                            var nonZeroCount = Popcnt.PopCount(nonZeroMask);
                            count += (int) nonZeroCount;
                        }
                    }
                }

                if (maxBatchIndex < boolArray.Length)
                {
                    for (int i = maxBatchIndex; i < boolArray.Length; i++)
                    {
                        if (boolArray[i])
                        {
                            count++;
                        }
                    }
                }
            }
            else
            {
                for (int i = 0; i < boolArray.Length; i++)
                {
                    if (boolArray[i])
                    {
                        count++;
                    }
                }
            }

            return count;
        }
    }

CountTruesBenchmarkOutput.md

BenchmarkDotNet=v0.11.5, OS=Windows 10.0.17763.529 (1809/October2018Update/Redstone5)
Intel Core i5-8350U CPU 1.70GHz (Kaby Lake R), 1 CPU, 8 logical and 4 physical cores
.NET Core SDK=3.0.100-preview5-011568
  [Host]     : .NET Core 3.0.0-preview5-27626-15 (CoreCLR 4.6.27622.75, CoreFX 4.700.19.22408), 64bit RyuJIT
  DefaultJob : .NET Core 3.0.0-preview5-27626-15 (CoreCLR 4.6.27622.75, CoreFX 4.700.19.22408), 64bit RyuJIT

Method	Mean	Error	StdDev
CountTruesFor	400.82 ns	2.0805 ns	1.7373 ns
CountTruesForEach	400.80 ns	2.7173 ns	2.4088 ns
CountTruesLinq	4,267.81 ns	23.5381 ns	22.0175 ns
CountTruesAvx2	29.96 ns	0.1736 ns	0.1450 ns