Odd. I didn't find it boring at all or slow. And that's also acknowledging that it isn't a hyper actively paced film like so many today try to be.
I think people find this boring, since they are used to the Marvel way of doing things. After TDK we've had a lot of Marvel movies, movies that are going at warpspeed, in hyperspace, on speed. That's not how Christopher Nolan makes movies. Never has, never will. Look at his previous work. He likes to take his time to tell a story, instead of going all Michael Bay on your ass. So, when he finally does come with the 'splosions and stuff, people freak out and find that the pacing is wrong. I can see how that can come across that way, but instead of filling up the entire movie, he's doing what movie-makers have been doing for decades: actually building up to something, telling a story, working towards this big build-up, which then delivers. And he accomplishes that be getting your attention with an action packed intro, which focusses you straight away. The thing is, after the Marvel-movies/Michael Bay era, we are used to not only getting an action-packed opening, but an action-packed everything.
I like how Nolan tells his story. I will, however, admit that his tactics worked better with TDK, where you really felt the suspense building throughout the movie. I did get that with TDKR, but less then I did with TDK.