My 2 year old 15" rMBP has no problems whatsoever driving an external 4K @ 60Hz monitor (over mini displayport), along with its own built in 2880x1800 display s

My 2 year old 15" rMBP has no problems whatsoever driving an external 4K @ 60Hz monitor (over mini displayport), along with its own built in 2880x1800 display simultaneously. By "no problem" I mean low power consumption and 2% CPU/GPU idle.