Its happened on everyone I've tried, various combinations. Its the reason I've waited so long to make the jump to retina. Thankfully the 2016 13" is lag free 🙂 Type "Retina UI lag" into google if you don't believe me. Some people don't notice the frame rate drops, so if you have not noticed it be thankful.Strange. I've never experienced it. Which iGPU and which version of MacOS?
Maybe there's things running the background slowing it down.