I see slowdowns on many places in the game. Firstly - and most noticeably - are the slowdowns on high-resolution FMV sequences of tracks, on "choose-the-track" menu in Arcade mode. What was so far non-glitch smooth ride, now it's an "drop-a-frame-to-kill-the-bird" thing. Very disappointing.
Secondly, there are lots of slowdowns on the tracks during drive. Notice that when I say "lots" it does not mean "dozens per track". It just means that there was NEVER ANY slowdowns in GT games EVER, and that famous glitch on Midfield in GT Concept was not an excuse, but just a dark-messenger of things to come. Glitches are often just on one place on the specific track, and manifested as classical frame-drops, and they last no longer then second, but they appear from time to time on lots of tracks. The biggest I saw so far is on Laguna Seca, just after the Corkscrew section, when you heading towards the last two corners.
Another graphical glitch is appearing even more often, particularly on the beginnings of the races with running-start, or in short-films after the races (first one is noticeable in the replays too) - it is manifested trough losing some of the horizontal lines in view, resulting in very jagged picture. It lasts for second or two, and the disappears, like the picture stabilises itself.
Both glitches could be result of incorporating high-definition views in graphic code, but I find them as bad thing for the most sophisticated GT game so far. Although, I tend to be very realistic and take into concern the limitations of PS2, together with all the details that GT4 is bringing - those glitches do not compromise gameplay in any way, but is just unusual and unpleasant (and that is the right word) to see them in GT game.
And one more thing about the tracks - since this thread is about tracks - I'm very disappointed with "reverse" tracks, since some of my favourites such as Grand Valley Reverse has lost that beautiful "afternoon" setting. I've tried all the "original" tracks I have unlocked, and all of them have the same lighting setting on both versions. I do not like that at all.
All tracks on demos I've played (all GT4 demos so far) were lot smoother on demos. Even the Nurb, which amazed me. Nurb on demo didn't have any glitches. In GT4 it does, just after the first section of corners. Small one, but noticeable. Which is the shame for the game.