CPU Increase when enabling AES-GCM

Ah, so the problem isn't so much AES-GCM but rather SHA384 or SHA256 as the MAC. Indeed, you'll see similar results with ECDHE-RSA-AES256-SHA384 and ECDHE-RSA-AES128-SHA256. Quick work-around is use "ECDHE+AES+SHA" in the cipher string.