Yeah there are definitely some times where some parts of the interface just totally stop responding for like 10 seconds, and then comes back to life. But the weird thing is that (for me at least) it's only specific parts that stop working, while the rest of the phone keeps responding, albeit slowly and stuttery.
For example, on the spring board I tapped on a folder to open it, but nothing happened. I tapped on it a bunch of times, and nothing happened. BUT, during this time I was able to swipe between different pages on the springboard perfectly fine. Weird stuff.
For people who need their phone to be immediately ready to instantly work for important usages, I'd recommend not installing iOS 8 yet, wait until the the 8.0.1 bug fix update comes out.