Thanks for your help Maciej. With your information I was able to come up with the following solution which is lightening fast:
public void find_and_replace_text(string curr_text, string new_text)
{
Word.Find findObject;
var missing = Type.Missing;
findObject = pDoc.Content.Find;
findObject.ClearFormatting();
findObject.Text = curr_text;
findObject.Replacement.ClearFormatting();
findObject.Replacement.Text = new_text;
object replaceAll;
replaceAll = Word.WdReplace.wdReplaceAll;
findObject.Execute(ref missing, ref missing, ref missing, ref missing,
ref missing,
ref missing, ref missing, ref missing, ref missing, ref missing,
ref replaceAll, ref missing, ref missing, ref missing, ref missing);
}