I assume you are using a .Net TextBox or RichTextBox. There are known performance issues with AsYouType spell checking in these controls that get worse as the number of error underlines visible on screen increases. Try using the UltraFormattedTextEditor if you are not already using it. Some performance enhancements were made for specifically spell checking that control. You can also submit the issue if you don't want to switch controls or the problem still exists after switching: http://devcenter.infragistics.com/Protected/SubmitSupportIssue.Aspx.