Program.cs

using System;
using System.Runtime.CompilerServices;
using BenchmarkDotNet.Attributes;
using BenchmarkDotNet.Running;

class Program
{
    static void Main(string[] args)
    {
        BenchmarkRunner.Run<Tests>();
    }
}

[MemoryDiagnoser]
public class Tests {
    [Benchmark]
    public int Span() {
        Span<bool> span = stackalloc bool[256];
        for (var i = 0; i < span.Length; i++) {
            if (i % 2 == 0) {
                span[i] = true;
            }
        }

        int count = 0;
        for (var i = 0; i < span.Length; i++) {
            if (span[i]) count++;
        }
        return count;
    }

    [Benchmark]
    [SkipLocalsInit]
    public int Span_ElseInit() {
        Span<bool> span = stackalloc bool[256];
        for (var i = 0; i < span.Length; i++) {
            if (i % 2 == 0) {
                span[i] = true;
            }
            else {
                span[i] = false;
            }
        }

        int count = 0;
        for (var i = 0; i < span.Length; i++) {
            if (span[i]) count++;
        }
        return count;
    }

    [Benchmark]
    [SkipLocalsInit]
    public int Span_ClearInit() {
        Span<bool> span = stackalloc bool[256];
        span.Clear();
        for (var i = 0; i < span.Length; i++) {
            if (i % 2 == 0) {
                span[i] = true;
            }
        }

        int count = 0;
        for (var i = 0; i < span.Length; i++) {
            if (span[i]) count++;
        }
        return count;
    }
}

Results.md

BenchmarkDotNet=v0.12.1, OS=ubuntu 20.04
Intel Core i5-2400 CPU 3.10GHz (Sandy Bridge), 1 CPU, 4 logical and 4 physical cores
.NET Core SDK=5.0.101
  [Host]     : .NET Core 5.0.1 (CoreCLR 5.0.120.57516, CoreFX 5.0.120.57516), X64 RyuJIT
  DefaultJob : .NET Core 5.0.1 (CoreCLR 5.0.120.57516, CoreFX 5.0.120.57516), X64 RyuJIT

Method	Mean	Error	StdDev	Gen 0	Gen 1	Gen 2	Allocated
Span	410.0 ns	2.87 ns	2.69 ns	-	-	-	-
Span_ElseInit	407.9 ns	2.39 ns	2.23 ns	-	-	-	-
Span_ClearInit	429.9 ns	2.80 ns	2.34 ns	-	-	-	-