RichTextBox loads very slow when locale is Hebrew

I have a very simple .NET C# program with a Rich text box.

When PC's Region Settings are set to UK (Language for non-Unicode Programs) I can load 500,000 characters is less than 1 second.

If I change Region Settings to Hebrew/Israel the same code takes around 50 seconds.

rtb.Text = sLongString;

rtb is a Rich Text Box and sLongString is a string containing plain text A-Z and \r\n