I can't think of a reason why it would show such a significant slow-down for you when you use a bold or underline character. When I use the same font and script that you mention in your post, I experience a small slow-down here, which is expected, both when scrolling and marking/copying text. However it's nowhere near several seconds. Which version of Windows are you using? Is it the English language version? What kind of CPU do you have?
