The threads with a bunch of Youtube videos, has always loaded slow me as well. What happens is when you open that particular page, it must load the image on the Video link freshly ever time, rather than it already existing. This is what slows the loading process. Depending on how fast your computer's processors is will likely affect how fast it loads, or even your internet speed. I do notice that there's the "white translucent checker background" that bleeds through if you scroll really fast on those pages too, because it's having trouble loading everything. I just hope the new design wouldn't be adding to the cause, and was worried about it myself. However, I just remember that it's always been like this for me.