Cycle is slowing down a lot with high def renders since Blender 2.70 in CUDA/GPU _and_ CPU.
Reducing the size of the titles seems to help a bit... But it's not an ideal solution because of slow render time with small thead on GPU
I have done several test renders with this scene:
Blender 2.77 RC1 --> "Render Preset at 100% (10240*4320), tiles size 128*128, CUDA/GPU" --> estimated time 2h22 (to long to finish the render)
Blender 2.77 RC1 --> "Render Preset at 100% (10240*4320), tiles size 64*64, CUDA/GPU" --> estimated time 1h47 (to long to finish the render)
Blender 2.70a --> "Render Preset at 100% (10240*4320), tiles size 128*128, CUDA/GPU" --> estimated time 3h (to long to finish the render)
Blender 2.70a --> "Render Preset at 100% (10240*4320), tiles size 128*128, CPU" --> estimated time 2h (to long to finish the render)
Blender 2.68a --> "Render Preset at 100% (10240*4320), tiles size 64*64, CPU" --> render time 11m50s (2*Xeon 2660v3)
Blender 2.68a --> "Render Preset at 100% (10240*4320), tiles size 128*128, CUDA/GPU" --> render time 3m54s
Blender 2.68a --> "Render Preset at 100% (10240*4320), tiles size 400*400, CUDA/GPU" --> render time 2m51s
PC Configuration:
Windows 8.1
2x geforce Titan + geforce 780
CPU 5820k
64Gb ram