Understanding Parallel.ForEachAsync vs Task.WhenAll in .NET

When you need to handle many items at the same time in .NET, two common options are Parallel.ForEachAsync and Task.WhenAll. Both run tasks in parallel, but they manage concurrency differently — and that difference can greatly affect performance.

Let’s look at how each one works and compare them with a real-world example.

The source code can be downloaded from GitHub.  Tools that I have used

1.        VS 2026 Insider

2.       .NET 8.0

3.       Console App

Parallel.ForEachAsync: Controlled Parallelism

Parallel.ForEachAsync (introduced in .NET 6) provides built-in throttling via MaxDegreeOfParallelism. It schedules work intelligently, without creating a separate task for every item.

Example

await Parallel.ForEachAsync(data, new ParallelOptions
{
    MaxDegreeOfParallelism = Environment.ProcessorCount
}, async (item, token) =>
{
    await ProcessItemAsync(item);
});

Key Idea
Runs only a limited number of iterations in parallel — typically one per CPU core.

Task.WhenAll: Fire-and-Wait for All Tasks

Task.WhenAll simply runs all tasks at once and waits until every one of them completes.

Example

var tasks = data.Select(item => ProcessItemAsync(item));
await Task.WhenAll(tasks);

Key Idea
Starts one task per item, no throttling — great for small workloads, but dangerous at scale.

Custom Throttled: Task.WhenAll – using SemaphoreSlim to limit concurrency for async workloads

static async Task ForEachAsync<T>(
        IEnumerable<T> source,
        int maxDegreeOfParallelism,
        Func<T, Task> action)
{
    using var semaphore = new SemaphoreSlim(maxDegreeOfParallelism);

    var tasks = source.Select(async item =>
    {
        await semaphore.WaitAsync();
        try
        {
            await action(item);
        }
        finally
        {
            semaphore.Release();
        }
    });

    await Task.WhenAll(tasks);
}
//usage:
Usage:
  var boundedTime = await MeasureTimeAsync(async () =>
  {
      await ForEachAsync(data, maxDegreeOfParallelism: 50, SimulateWorkAsync);
  });

Observations from Your Benchmark

Method	10,000 items	100,000 items	Notes
Parallel.ForEachAsync	78.43s	782.79s	Very slow because concurrency is limited to Environment.ProcessorCount (e.g., 8). Great for CPU-bound tasks, but for async I/O it’s throttling too much.
Task.WhenAll	1.11s	15.04s	Extremely fast because all 10K or 100K tasks run concurrently. Ideal for async I/O. Memory usage is high but delay is very small.
Custom Bounded (SemaphoreSlim)	12.97s	131.28s	Middle ground. Controlled concurrency (e.g., 50 tasks at a time). Prevents thread pool overload while still allowing high concurrency.

Why the Numbers Look This Way?

1. Parallel.ForEachAsync

   • Limited by MaxDegreeOfParallelism = CPU count (~8 on most machines)

   • Each task waits 50ms (simulated I/O) before completing

   • So 10,000 / 8 × 50ms ≈ 78s — matches your result

2. Task.WhenAll

   • Launches 10,000 tasks immediately

   • Task.Delay is non-blocking → tasks don’t consume threads

   • Finishes in ~1s (10K) and 15s (100K)

3. Custom Bounded

   • Limited concurrency (50 in your example)

   • 10,000 / 50 × 50ms ≈ 10s — matches closely (12.97s)

   • 100,000 / 50 × 50ms ≈ 100s — matches closely (131.28s)

Key Takeaways

• CPU-bound tasks → Parallel.ForEachAsync wins

• Async I/O tasks with thousands of operations → Task.WhenAll is fastest, but can risk memory pressure

• Large async workloads with controlled concurrency → Custom SemaphoreSlim approach is safest

Note: Adjust maxDegreeOfParallelism in your custom method depending on CPU cores and I/O type

 Conclusion

Task.WhenAll consistently outperforms Parallel.ForEachAsync and the custom bounded implementation by a significant margin, especially as the number of items increases. Parallel.ForEachAsync shows the worst performance, likely due to its unbounded concurrency and overhead per iteration. The custom bounded approach offers a middle ground, limiting concurrency to reduce resource contention, resulting in much better performance than Parallel.ForEachAsync but still slower than Task.WhenAll. Overall, Task.WhenAll is the most efficient approach for high-concurrency async operations in this scenario.

Happy Coding!