C# winform to read multi files in a folder(1400 files, 1-2kb each). My code runs extremly slow, costs more than 30 minutes. How can I improve?

string[] files = Directory.GetFiles(path, "*.txt");//get the path foreach (string file in files) { string text = File.ReadAllText(file);//read all files textBox1.Text += text + " ";//show result in textbox }