https://www.reddit.com/r/csharp/comments/cu1k4m/some_c_skills_important_for_junior_developers/exqq0db/

Program.cs

using System.Collections.Generic;
using System.Linq;
using BenchmarkDotNet.Attributes;
using BenchmarkDotNet.Running;

public class Program
{
    static void Main() => BenchmarkRunner.Run<Benchmark>();
}

[MemoryDiagnoser]
public class Benchmark
{
    const int N = 1000;

    private List<List<int>> lotsOfStuff =
        Enumerable.Range(0, N)
            .Select(i => Enumerable.Range(0, N).ToList())
            .ToList();

    [Benchmark]
    public int Linq()
    {
        return lotsOfStuff
            .SelectMany(z => z)
            .Aggregate(0, (a, b) => a + b);
    }

    [Benchmark]
    public int Imperative()
    {
        int sum = 0;
        
        foreach (var collection in lotsOfStuff)
        {
            foreach (var item in collection)
            {
                sum += item;
            }
        }

        return sum;
    }    
}

results.md

// * Summary *

BenchmarkDotNet=v0.11.5, OS=Windows 10.0.18362
Intel Core i5-2300 CPU 2.80GHz (Sandy Bridge), 1 CPU, 4 logical and 4 physical cores
.NET Core SDK=3.0.100-preview8-013656
  [Host]     : .NET Core 3.0.0-preview8-28405-07 (CoreCLR 4.700.19.37902, CoreFX 4.700.19.40503), 64bit RyuJIT
  DefaultJob : .NET Core 3.0.0-preview8-28405-07 (CoreCLR 4.700.19.37902, CoreFX 4.700.19.40503), 64bit RyuJIT

Method	Mean	Error	StdDev	Gen 0	Gen 1	Gen 2	Allocated
Linq	18.981 ms	0.1206 ms	0.1069 ms	-	-	-	40104 B
Imperative	3.473 ms	0.0191 ms	0.0159 ms	-	-	-	-

// * Hints *
Outliers
  Benchmark.Linq: Default       -> 1 outlier  was  removed (19.53 ms)
  Benchmark.Imperative: Default -> 2 outliers were removed (3.53 ms, 3.55 ms)

// * Legends *
  Mean      : Arithmetic mean of all measurements
  Error     : Half of 99.9% confidence interval
  StdDev    : Standard deviation of all measurements
  Gen 0     : GC Generation 0 collects per 1000 operations
  Gen 1     : GC Generation 1 collects per 1000 operations
  Gen 2     : GC Generation 2 collects per 1000 operations
  Allocated : Allocated memory per single operation (managed only, inclusive, 1KB = 1024B)
  1 ms      : 1 Millisecond (0.001 sec)