In order to download large files in ASP.NET MVC efficiently, you must maximize memory use, avoid timeouts, and guarantee a seamless user experience. The best methods are listed below:

1. Make use of FileStreamResult, which is effective for big files.
By doing this, the file is streamed rather than fully loaded into memory.

public ActionResult DownloadLargeFile()
{
string filePath = Server.MapPath("~/Files/LargeFile.zip"); // Adjust file path
string fileName = "LargeFile.zip";

if (!System.IO.File.Exists(filePath))
{
    return HttpNotFound("File not found.");
}

var stream = new FileStream(filePath, FileMode.Open, FileAccess.Read);
return File(stream, "application/octet-stream", fileName);
}

Why?
Streams the file without consuming memory.
Works well for large files (e.g., >1GB).

2. Use Response.Write to Stream File in Chunks
This method writes file in chunks to prevent excessive memory usage.
public void StreamLargeFile()
{
string filePath = Server.MapPath("~/Files/LargeFile.zip");

if (!System.IO.File.Exists(filePath))
{
    Response.StatusCode = 404;
    Response.End();
    return;
}

Response.Clear();
Response.ContentType = "application/octet-stream";
Response.AddHeader("Content-Disposition", "attachment; filename=LargeFile.zip");

const int bufferSize = 1024 * 1024; // 1MB chunks
byte[] buffer = new byte[bufferSize];

using (FileStream fileStream = new FileStream(filePath, FileMode.Open, FileAccess.Read))
{
    int bytesRead;
    while ((bytesRead = fileStream.Read(buffer, 0, buffer.Length)) > 0)
    {
        Response.OutputStream.Write(buffer, 0, bytesRead);
        Response.Flush(); // Sends the chunk immediately to the client
    }
}

Response.End();
}

Why?
Sends 1MB chunks, preventing high memory usage.
Flushes data after each chunk to avoid server timeouts.

3. Asynchronous Streaming (Recommended for Web API)
For MVC + Web API applications, use an async stream for optimal performance.
public async Task<IActionResult> DownloadLargeFileAsync()
{
string filePath = Path.Combine(Directory.GetCurrentDirectory(), "wwwroot", "Files", "LargeFile.zip");

if (!System.IO.File.Exists(filePath))
{
    return NotFound("File not found.");
}

var stream = new FileStream(filePath, FileMode.Open, FileAccess.Read, FileShare.Read, bufferSize: 4096, useAsync: true);
return File(stream, "application/octet-stream", "LargeFile.zip");
}

Why?
Asynchronous file streaming prevents thread blocking.
useAsync: true optimizes I/O operations.

4. Enable Large File Downloads in Web.config
Modify web.config to allow large downloads:
<system.webServer>
<security>
    <requestFiltering>
        <requestLimits maxAllowedContentLength="2147483648" /> <!-- 2GB -->
    </requestFiltering>
</security>
</system.webServer>

Why?
Increases max file size limit (default is 30MB in IIS).

5. Use Content-Disposition Header (For Better Browser Support)
If the filename contains special characters, encode it properly:
public ActionResult DownloadLargeFile()
{
string filePath = Server.MapPath("~/Files/LargeFile.zip");
string fileName = "Large File.zip";

var stream = new FileStream(filePath, FileMode.Open, FileAccess.Read);
var cd = new System.Net.Mime.ContentDisposition
{
    FileName = fileName,
    Inline = false // Forces download
};
Response.Headers.Add("Content-Disposition", cd.ToString());

return File(stream, "application/octet-stream");
}

Why?
Prevents filename issues (especially in Chrome & Edge).

6. Optimize IIS for Large Files
If you use IIS, increase timeout settings:

appcmd set config /section:serverRuntime /uploadReadAheadSize:10485760
appcmd set config /section:system.webServer/serverRuntime /maxRequestEntityAllowed:2147483648

Why?
Prevents timeouts for large file downloads.