Why Async and Await Improve Performance in .NET Core

Async and Await Improve Performance (Photo Credit — ChatGPT)

When building high-performance applications with .NET Core, two keywords appear everywhere:

async and await

But a common interview question is:

How exactly do async and await improve performance?

The short answer is that async and await do not necessarily make a single operation faster. Instead, they improve the scalability, responsiveness, and throughput of an application by avoiding blocked threads during I/O-bound operations.

Let’s understand this with a simple example.

The Problem With Synchronous Code

Imagine that we have an API endpoint that retrieves user information from a database.

public IActionResult GetUser(int id)
{
    var user = _dbContext.Users
                 .FirstOrDefault(x => x.Id == id);
    return Ok(user);
}

When this API receives a request, a thread starts executing the code.

When the application sends the query to the database, the thread waits until the database returns the result.

The flow looks like this:

Request arrives
      ↓
Thread starts processing
      ↓
Database query starts
      ↓
Thread waits...
Thread waits...
Thread waits...
      ↓
Database result arrives
      ↓
Thread continues
      ↓
Response returned

The important point is that while the database is processing the query, the application thread is mostly waiting.

For one request, this may not be a serious problem.

But imagine 1,000 users calling the API at approximately the same time. If many request threads are blocked while waiting for database or network responses, the application can experience thread-pool pressure and reduced throughput.

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How Async and Await Help

Now consider the asynchronous version:

public async Task<IActionResult> GetUser(int id)
{
    var user = await _dbContext.Users
                               .FirstOrDefaultAsync(x => x.Id == id);
    return Ok(user);
}

When execution reaches:

await _dbContext.Users.FirstOrDefaultAsync();

the database operation starts asynchronously.

While the database operation is in progress, the request does not need to keep a thread blocked just waiting for the result.

Conceptually, the flow becomes:

Request arrives
      ↓
Thread starts processing
      ↓
Async database query starts
      ↓
Thread returns to the thread pool
      ↓
Thread can process other work
      ↓
Database operation completes
      ↓
Continuation is scheduled
      ↓
Method continues
      ↓
Response returned

This is the main advantage of asynchronous programming in web applications.

Does Async/Await Make the Database Query Faster?

No.

This is one of the most important points to understand. Suppose a database query takes 3 seconds.

A synchronous call may still take approximately 3 seconds:

var data = GetData();

An asynchronous call may also take approximately 3 seconds:

var data = await GetDataAsync();

The database operation itself has not magically become faster.

The difference is in how efficiently the application uses its threads while waiting.

With synchronous code, a thread may remain blocked during the wait.

With asynchronous I/O, the application can avoid tying up that thread for the entire waiting period, allowing server resources to be used more efficiently.

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A Restaurant Analogy

Think about a waiter in a restaurant.

In a synchronous model, the waiter takes an order, gives it to the kitchen, and then stands near the kitchen doing nothing until the food is ready.

Only after the food is ready does the waiter serve the customer and move to another table.

That is inefficient.

In an asynchronous model, the waiter takes an order and sends it to the kitchen. While the food is being prepared, the waiter serves other tables.

When the kitchen finishes the food, the waiter comes back and continues the original task.

The kitchen does not cook the food faster.

The waiter simply uses the waiting time more efficiently.

That is similar to how async and await help server applications handle I/O-bound work.

Where Should We Use Async/Await?

Async and await are particularly useful for I/O-bound operations such as:

1. Database operations

var users = await _dbContext.Users.ToListAsync();

1. Calling external APIs

var response = await httpClient.GetAsync(requestUri);

1. Reading files asynchronously

var content = await File.ReadAllTextAsync(filePath);

1. Writing files asynchronously

await File.WriteAllTextAsync(filePath, content);

1. Network communication and other naturally asynchronous I/O operations.

These operations spend significant time waiting for something outside the current CPU execution flow.

Async/Await Is Not Automatically Better for CPU-Bound Work

Consider a CPU-intensive calculation:

public int Calculate()
{
    int result = 0;
    for (int i = 0; i < 1_000_000; i++)
    {
        result += i;
    }
    return result;
}

Simply adding async and await does not make this calculation faster.

Async programming is mainly beneficial when the application spends time waiting for I/O.

For CPU-bound work, different techniques may be appropriate depending on the application, such as parallel processing, background workers, queues, or carefully using multiple cores.

Why Is Async/Await Important in Web APIs?

Web APIs often perform many I/O operations:

API Request
    ↓
Database Query
    ↓
External API Call
    ↓
Blob Storage Operation
    ↓
Message Queue Operation
    ↓
API Response

If all these operations block threads, an application can struggle under high concurrency.

Using asynchronous APIs throughout the request path can improve:

• Scalability
• Throughput
• Thread utilization
• Responsiveness under concurrent load
• Ability to handle many simultaneous requests

Common Mistake: Blocking Async Code

Avoid patterns like:

var result = GetDataAsync().Result;

or:

GetDataAsync().Wait();

These patterns synchronously block while waiting for asynchronous work and can remove much of the scalability benefit of async programming.

Prefer:

var result = await GetDataAsync();

A useful principle is often called async all the way: if a lower-level operation is asynchronous, allow the calling methods to remain asynchronous through the call chain whenever practical.

Common Mistake: Using Async Without Await

This method is marked async, but it does not await asynchronous work:

public async Task<int> GetNumber()
{
    return 10;
}

The async keyword itself does not create performance improvements.

The benefit comes from correctly using asynchronous APIs for operations that can actually complete asynchronously.

Final Takeaway

The most important concept is:

Async and await do not necessarily make an individual operation faster. They help an application use threads more efficiently while waiting for I/O operations, which can improve scalability and throughput under concurrent load.

For interview purposes, a strong short answer is:

In .NET Core web applications, async and await improve scalability by avoiding blocked request threads during I/O-bound operations such as database queries, HTTP calls, and file operations. While an I/O operation is pending, the thread can return to the thread pool and process other work. This improves thread utilization and allows the application to handle more concurrent requests, although it does not necessarily make the individual I/O operation itself faster.

Understanding this distinction between speed and scalability is the key to understanding asynchronous programming in .NET.