Afaik, 2016+ 13 touch bar models have a faster integrated graphics cards than the 15. So if dGPU isn't being used (e.g. no specific processes, no external monitors), 15 will have inferior UI performance, on top of 15 needing to drive the higher res display.
With that said, I think the integrated GPU on the 15 should be enough to run macOS UI smoothly. If you see noticeable lag, it must be software related. I remember reading about this issue on many previous models and there was some potential fix that involved deleting some files.