I don't like viewing in portrait mode because everything is so much smaller and squished together. There's nothing wrong with the speed though, it's the exact same as landscape, except longer, and not as wide.
This is funny, 'cos when I had the Galaxy Tab I did everything in Portrait mode. Except watching movies/youtube of course.
But with the PB it just feels so right in the hand in landscape, I never bother with portrait mode. Also since I got this cheap case/stand which opens in landscape makes it great for watching or reading.