I bought a 13 inch rmbp last year and returned it due to really annoying UI lag. This year after being really disappointed with the new offerings I ended up purchasing a 15 inch refurbished model from last year. Came in a new box, everything works well and I have upgraded to Sierra. I wanted to ask if the UI lag is still an issue for people with the base model? It isn't drastic, mission control etc seem to be smooth, but resizing certain windows tend to be choppy. It really is inexcusable for a machine this expensive with a quad core CPU and decent integrated graphics. From what I have read people with dedicated graphics do not have this problem at all. I love the machine overall but the choppiness at times ruins the experience a little.