I haven't had any UI lag, but I've had battery life issues with my 2015 13-inch rMBP like you. I was getting about 7 hours with brightness around 75%. Activity monitor showed nothing hogging the CPU. Many professional reviews (CNET, Laptopmag.com, PCMag.com, others) report well over 10 hours per charge.
I had a Genius Bar appointment yesterday to discuss the issue. They did diagnostics which showed no problems with the battery. They gave the standard line, many variables affect battery life and it's not at all unusual to experience 2 fewer hours of battery life than Apple's 10-hour rating. Fine, but if that's the case, why is just about every professional reviewer getting well over 10 hours? Frustrating.
I'm going to try reducing the display brightness to 50-60% (from 75%), which I expect will net me an extra hour. I don't absolutely NEED 10 hours, I'm rarely using the machine away from an outlet for more than 3 hours. When I do, I'll just be sure to keep the display brightness as low as possible, and the keyboard backlight off or low.
Good luck! And, especially, good luck with the UI lag - that sounds like the bigger problem. Sorry these have taken up so much of your time. Hope you get them resolved and can start enjoying your rMBP. Aside from my battery life issue, I am really enjoying the rMBP a lot and so happy I moved to it from a MB Air.