struct_benchmark.cs

using BenchmarkDotNet.Attributes;
using BenchmarkDotNet.Running;
using CommunityToolkit.HighPerformance;

public struct Position {
    public float x;
    public float y;
};

public class TestStructModification {
    public delegate void ActionRef<T>(ref T item);

    public List<Position> positions;
    
    [GlobalSetup]
    public void Setup() {
        positions = new List<Position>();
        for (var i = 0; i < 100_000; i++) {
            positions.Add(new Position { x = i, y = i });
        }
    }

    [Benchmark]
    public void NewWithIncrement() {
        Entrypoint(NewWithIncrement);
    }

    [Benchmark]
    public void NewWithAddition() {
        Entrypoint(NewWithAddition);
    }

    [Benchmark]
    public void UpdateInPlace() {
        Entrypoint(UpdateInPlace);
    }

    public void Entrypoint(ActionRef<Position> action) {
        var positionsSpan = positions.AsSpan();
        for (var i = 0; i < positions.Count; i++) {
            action(ref positionsSpan[i]);
        }
    }

    public void NewWithIncrement(ref Position position) {
        position = new Position {
            x = position.x++,
            y = position.y++
        };
    }

    public void NewWithAddition(ref Position position) {
        position = new Position {
            x = position.x + 1,
            y = position.y + 1,
        };
    }

    public void UpdateInPlace(ref Position position) {
        position.x += 1;
        position.y += 1;
    }
}

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

Benchmark written for genaray/Arch#49

Very surprising results:

|           Method |     Mean |   Error |  StdDev |   Median |
|----------------- |---------:|--------:|--------:|---------:|
| NewWithIncrement | 199.9 us | 3.34 us | 8.31 us | 197.0 us |
|  NewWithAddition | 194.7 us | 2.82 us | 2.35 us | 194.2 us |
|    UpdateInPlace | 193.0 us | 1.09 us | 0.85 us | 193.1 us |