That's kind of how it was for me. I thought the first half was a little bit slow and uneven, though I was still interested the whole way through. Once the plot was finally executed, I was pretty much riveted the entire way from that point on.What's weird is that the movie after that point worked for me. Watching the plan come together, yet unravel at the same time was entertaining and tightly paced from the bombing to the very end. It was the pacing earlier in the movie that seemed to be inconsistent.The pacing problems in my opinion were due to adherence to fact. The failure of Stauffenberg's attempt introduces an early anticlimax, not a ratcheting of tension. Screenwriters are notorious for rewriting history so that it plays better on screen. It seems we really expect variations on the old stories rather than genuine realism. Indeed, we are so dominated by our stylistic expectations that we can reject it because it so ungainly and unartistic.
Very good movie, overall. It's too bad Cruise has such a poor reputation these days, I kind of feel like Valkyrie went a bit overlooked because of his involvement.