RichTextBox slows down the entire UI

I have an application which opens multiple text files in different tabs. I use RichTextBox while processing this data in my application. However, loading large text files(10MB) slows the entire UI. Is there a way to solve this problem? I am thinking of lazy loading the file, so when the user presses the scroll button, later part of the file has to be loaded. Is there a place where I can start with this approach? Please help me I am pretty new to WPF and C# programming.

RichTextBox mcRTB = new RichTextBox();                    
rtbList.Add(mcRTB);
mcRTB.SelectionChanged += mcRTB_SelectionChanged;

TabItem tab = new TabItem();                 

Paragraph paragraph = new Paragraph();
paragraph.Inlines.Add(System.IO.File.ReadAllText(filePath));                       
FlowDocument document = new FlowDocument(paragraph);

mcRTB.Document = document;
                       
StringBuilder dataContent = new StringBuilder();   
//this statement takes few seconds, need to find alternatives                    
dataContent.Append(new TextRange(mcRTB.Document.ContentStart, mcRTB.Document.ContentEnd).Text);      

mcRTB.TextChanged += new System.Windows.Controls.TextChangedEventHandler(SegFileDataContentChanged); 
tab.Content = mcRTB;