Add a RadSpellchecker and a RadTextBox:

        public RadForm1()
        {
            InitializeComponent();

            this.radSpellChecker1.AutoSpellCheckControl = this.radTextBox1; 
        }

When running the application and typing fast "th", you will notice that there is a slight delay before seeing "h" in the editor.

However, the next time you type "th", it will be entered instantly.

All the dictionaries for the spell checking functionality are loaded with MEF. That is why the initial loading may be slow. RadRichTextEditor uses internally Telerik.WinForms.Documents.Proofing.DocumentSpellChecker which offers AutomaticallyLoadDefaultDictionaries property. It controls whether MEF should be used to load default dictionaries. Such functionality should be introduced for Telerik.WinControls.SpellChecker.Proofing.DocumentSpellChecker used by RadSpellchecker.

Note: Please refer to Example 8: Use Spell Checking Without MEF: https://docs.telerik.com/devtools/wpf/controls/radrichtextbox/mef