Metal is not supported on rMBP's with dGPU's when UI animations are concerned, even when the iGPU is engaged. Will Apple ever implement Metal on these systems when using the iGPU or dGPU for UI animations? Has there been any talk of this? If Metal is not implemented on these machines, then iGPU-only Macs (i.e the cheaper, less powerful Macs) will always have the upper hand at UI fluidity (Mission Control, resizing windows etc). This is pretty strange to me, and a little unfair if you think about it. In my experiences, El Capitan isn't much smoother than Yosemite, which may be due to the lack of Metal. FWIW, I'm running the late 2013 15" rMBP with Iris Pro and GT 750m.