Except they're not. Most of the time though, the bar will come up instantly, but the actual app thumbnails lag behind. It works better this way anyway since the app you're trying to switch to, is more than likely one of the first three that appear.
Still a massive improvement over ICS, which I felt was good enough (until you had upwards of 10 or more apps open).
So, having had JB for three days now, these are my findings:
Frame rate STAYS at 60fps when the keyboard is present. This is a HUGE deal for the typing experience.
The multitasking bar is faster, and it almost always scrolls at 60fps (slight drops with an intensive application running in the foreground). Swiping apps away is always 60fps.
Live wallpapers still suck. Phase beam is 60fps though (probably because it doesn't scroll)
Browser has reached performance parity with the iOS safari on the iPad 3. It's scary smooth, even with flash playing. Gifs are still choppy, but have no effect on scrolling or what have you. Also, the address bar doesn't cause skipped frames when it appears (from scrolling up) and the multi window view to switch between "tabs" is 100% 60fps. Websites that used to cause frame drops (on ios as well), no longer do. Xda was a big culprit.
Home screen almost never drops frames, even when widgets are refreshing. Widgets almost never redraw either (I have every page filled).
App drawer animation is also 60fps 99% of the time. Glad to see this, since the button glow animation used to cause major stutters on ICS.
3rd party and even some google apps, need a bit of that project buttery love. The UI makes going to non JB native apps, incredibly painful. Overall, the GN is actually a bit more consistent in performance than my iPad 3! Still not 4s, ipad2, or wp7 smooth, but I'd say it's 90% there (ICS 80%).
One thing I've noticed about this new rendering engine. It waits for animations to pre buffer before opening the app, each time. What this means is that if the system is kinda busy, the app takes longer to actually launch, then it would on ICS. Although when it does, it's smooooooth.