Interestingly, my zooming is now slower than it used to be too... and the reason? I installed Yosemite... Mavericks was fast and slick at all times, but Yosemite seems to have slowed down something. So, I agree with you, it shouldn't happen, but this wasn't actually a problem that existed for our entire beta period and only appeared after launch. There's no reason for Yosemite to have slowed stuff down, but it has - I even see my safari window being slower at scrolling than it used to be in Mavericks. Hopefully Apple will fix this, but if not we can try to do something (although I genuinely can't think what - seeing as it seems to be completely out of our control)
My MacBook Pro is about 4 months old and has 8GB RAM - and I can safely say that it was utterly flawless in Mavericks.