Well, first of all, I obviously don't know until the reviews come up, but I what I can do is give you my best guess.
My guess is that it will work just fine. And this is why:
1. MacBook Pro Retina from 2012 uses an Intel HD 4000 to run the same resolution and the UI doesn't lag. There was some lag here and there, but it was fixed in later OS updates. I checked a few videos on YouTube (one example:) and it's working without lag. Intel HD 4000 is a weaker card than HD 530. As I said previously, GFLOPS isn't everything, but HD 4000 has 332.8 and HD 530 has 441.6. This is the main reason I don't think the UI will lag: it works fine on a weaker card at the same resolution (by the way, if you attach a secondary display it will always use the dGPU, so don't worry about that either).
2. You may disagree with this one, but for me this next one is also a good reason to believe it won't lag. I trust Apple to give us a good experience: Sure, there were Apple devices that did lag, I agree. But mostly it was due to either lack of software optimizations or hardware limitations of that time. I seriously doubt Apple would allow their flagship Mac to lag when doing something as basic as UI stuff - when there is more capable hardware available (in other words, they can decide to use less powerful components - as long as it doesn't take away from the experience, but not if it does).
Sure, there is this argument that they value portability over performance and that they would alow their computer to be underpowered to make it thinner and lighter. This is debatable to say the least, but even if it were true - they would never do it in an area where an average user would have a bad experience. And everyone would notice UI lagging.
For example, they will - for whatever reason, not going into it here - use a Skylake chip instead of Kaby Lake because, no matter how much people here whined about it - no one will notice the difference while using the computer. But the UI lagging - that will make the whole computer seem slow. And Apple is all about perception.
3. In my experience, and from what I've read - when the UI does lag, it's rarely, if ever, because of the GPU. Since we associate GPU with framerates and graphics we always attribute lag to it, but in reality, the UI usually lags because of some background process or data collecting.
Makes sense there, with how apple shouldn't be comprising UI performance. I guess we'll see soon!