Just like iOS 7 and 8, CarPlay has some very swoopy animations to jump in and out of the home screen and between apps. These Animations take time to complete that leads some people to feel CarPlay can be a little slow just like their opinion of iOS 7 and 8 in general.
Well just like iOS 7 and 8 if you turn on reduce motion in accessibility the transitions between apps in CarPlay dramatically improves as well. I'll make a video of it when I get off work, but suffice to say it's impressive and a welcome improvement.
Try it out. Settings-general-accessibility-reduce motion-on. It's nice.