Protgram.cs

using BenchmarkDotNet.Attributes;
using BenchmarkDotNet.Running;
using System.Numerics;

BenchmarkRunner.Run(typeof(ConcatBenchmark).Assembly);

[MemoryDiagnoser]
public class ConcatBenchmark
{
    private readonly int[] arr1 = new[] { 1, 2, 3, 4, 5, 0 },
        arr2 = new[] { 6, 7, 8, 9, 0 };

    [Benchmark]
    public int[] Linq() => arr1.Merge1(arr2);

    [Benchmark]
    public int[] CollectionExpression() => arr1.Merge2(arr2);
}

public static class ArrayExtensions
{
    public static T[] Merge1<T>(this T[] front, T[] back) where T : INumber<T>
        => front.Concat(back).ToArray();
    public static T[] Merge2<T>(this T[] front, T[] back) where T : INumber<T>
        => [.. front, .. back];
}

Results.txt

// * Summary *

BenchmarkDotNet v0.13.12, Windows 11 (10.0.23615.1000)
AMD Ryzen 9 7900X, 1 CPU, 24 logical and 12 physical cores
.NET SDK 8.0.100
  [Host]     : .NET 8.0.0 (8.0.23.53103), X64 RyuJIT AVX-512F+CD+BW+DQ+VL+VBMI
  DefaultJob : .NET 8.0.0 (8.0.23.53103), X64 RyuJIT AVX-512F+CD+BW+DQ+VL+VBMI


| Method               | Mean      | Error     | StdDev    | Gen0   | Allocated |
|--------------------- |----------:|----------:|----------:|-------:|----------:|
| Linq                 | 40.055 ns | 0.2545 ns | 0.2380 ns | 0.0110 |     184 B |
| CollectionExpression |  6.182 ns | 0.0426 ns | 0.0355 ns | 0.0043 |      72 B |