I ran two different benchmark tests to demonstrate different approaches for updating a WinForms TextBox. I built a simple form a rich text box. On separate runs, I filled the text box with text as shown below:

// The += approach:

richTextBox1.Text = string.Empty;
for (int i = 0; i < 1000; i++)
{
    richTextBox1.Text += "Hello, World!\r\n";
}

// The StringBuilder approach:

richTextBox2.Text = string.Empty;
var builder = new StringBuilder();
for (int i = 0; i < 1000; i++)
{
    builder.AppendLine("Hello, World!\r\n");
    richTextBox2.Text = builder.ToString();
}

The AppendText() approach

richTextBox3.Text = string.Empty;
for (int i = 0; i < 1000; i++)
{
    richTextBox3.AppendText("Hello, World!");
}

And the results on my old machine?

• += Approach: 10.7470703 seconds.
• StringBuilder: 10.7353515 seconds.
• AppendText(): 0.7041015 seconds.

I ran another benchmark. I filled a text box with 10MB of dummy text, then used the three approaches to append "Hello, World!" to the text box.

And the results of this?

• += Approach: 11.7470703 seconds.
• StringBuilder: 12.0615235 seconds.
• AppendText(): 0.1142578 seconds.

Conclusion: It is much faster to append to a text box using AppendText() than to assign text using string concatenation or from an intermediate data store such as s StringBuilder. Also note that the update rate drops substantially (9 per second) when the text box contained 10MB of text. Depending on the circumstances, a form designer might use multi-threading and caching (via StringBuilder) of text in chunks to reduce the frequency of updates required in the text box.

Note that the .NET text box is a wrapper of the Win32 text box which doesn't use immutable strings, thus giving it this edge in appending performance.