Minimal API Performance: Source Generators vs Reflection

.NET 8’s source generators for JSON serialization eliminate reflection at runtime. I wanted to know if it mattered for our gateway’s hot path.

Benchmark Setup

[MemoryDiagnoser]
public class SerializationBenchmark
{
    private readonly Order _order = OrderFactory.CreateLarge();

    [Benchmark(Baseline = true)]
    public string ReflectionSerialize() =>
        JsonSerializer.Serialize(_order);

    [Benchmark]
    public string SourceGenSerialize() =>
        JsonSerializer.Serialize(_order, OrderContext.Default.Order);
}

Results

Method	Mean	Allocated
Reflection	842 μs	48.2 KB
Source Gen	267 μs	12.1 KB

3.2x faster, 75% less allocation.

When It Matters

• High-throughput APIs (>1K req/s per instance)
• Large or nested DTOs
• Latency-sensitive paths

When It Doesn’t

• Low-traffic internal APIs
• Prototyping / MVPs
• DTOs that change frequently (source gen adds compile-time coupling)

We adopted source generators for all gateway request/response models. Compile time increased ~2 seconds — negligible.