Gosh, I envy your specs 
I started an Immense, played 18 turns (in 1 hour), zero problems. I can zoom all the way out and drag the map arbitrarily, with only momentary pauses.
Alienware M17X R2, 8 GB, ATI Mobility Radeon 5760x1 or 2 (the Crossfire may have failed), with DisplayPort output to a Dell UltraSharp at 1200p, and HDMI output to a Sony soundbar.
Windows 8.1, 240 GB SSD.
Zero problems anywhere. I can even slide over to Excel 2013 or Firefox on my laptop display, with no problems. The GC3 music correctly ceases while Excel has foreground focus, then resumes seamlessly when I click back on GC3.
Heck, I can fire up my HDMI pass-thru to my FullHD projector and watch a stream of a sports event at 1080p, while playing GC3 (or Sorcerer King) at 1200p, and running Excel + a gamecast browser window on the built-in monitor, with no problems. The stream audio merges with the GC3 music. (I invade Iridia and the pbp guy screams "goool! nnggg-ooooool")
WAG: It's your Windows 7/DX?