Render times on an older machine

I’m running WIn10 on a core i7 870 2.93GHz with 16Gb and AMD HD6900 Series GPU 4GB DDR3.

I found no difference when choosing to render with GPU, seemed to use CPU regardless of settings as when I checked computer performance, in all tests CPU reported to run at 109% (The Device setting in Blender was also greyed out although I could change it)
CPU (16x16): 10:30
CPU (64x54): 8:40
CPU (128x128): 8:00
CPU (256x256): 6:36
CPU (512x512): 8:36
CPU (1024x1024) 11:15
GPU (128x128): 7:35
GPU (256x256): 6:30 - cpu?
GPU (512x512): 11:25
GPU (1024x1024): 40:00

Chances are your AMD card is not CUDA compatible, so no advantage.
http://www.amd.com/en-us/products/graphics/desktop/6000/6990#

Ha, you think your computer is old! Try the one I’m using! Actually, with CUDA, the old CPU does not seem to matter much (unless more complex scenes requiring RAM):

CPU:
Intel Core Duo 6600 @ 2.4Ghz x2
OS Lubuntu 16.04 LTS
GPU:
Dual GeForce GTX 750 Ti’s:

TL;DR:
GPU’s won handily over CPU (this was by design, special purchased for Blender)
Res-matched tile sizes beat out square tiles by a few seconds, which can add up.

In other words, for BMW27 benchmark scene:

Dual GPU’s won easily (10x’s as fast as CPU) with res-matched tile size:
240x135 *1:58.09
second best for GPU was using square tiles:
256x256 **2:00.53

best tile size for CPU was:
15x8 *23:26.63
second best for CPU was square tiles:
16x16 **23:30.97

=========Details============
Nominal 1920x1080
rendered at 50% resolution:

Square Tiles:

8x8     GPU   60+min(est)   CPU   23:35.28
16x16   GPU   17:17.62      CPU **23:30.97          
32x32   GPU    5:53.30      CPU   23:36.28
64x64   GPU    2:40.69      CPU   24:07.87
128x128  GPU   2:03.13
256x256  GPU **2:00.53      CPU   25:45.01
512x512  GPU   2:38.47      CPU   29.06.47
1024x1024 GPU  4:56.20      CPU   47+min (est)

Res Matched for rendering at 50% resolution
using following derived by
dividing full nominal full resolution by 256 to 2:

1920x1080 div    
by   CPU
/256 8x8   (23:35.28 above)
/128 15x8  *23:26.63
/64  30x17  23:32.84
/32  60x34  23:34.08

by GPU: 
/16  120x68    2:17.96     
/8   240x135  *1:58.09
/4   480x270   2:01.22

* = best in category (CPU vs GPU).
** = 2nd best in category (CPU vs. GPU) Res un-matched had slight disadvantage

Another thing to check is whether the Cycles Compute Device is actually enabled in User Preferences.
When I did my first tests I got ridiculously long render times as well e.g GPU 256/256 took around 6 mins and my device setting was also greyed out in Blender.
So I went to the user preferences again to double check and realised I actually had to click on my graphics card text! (possibly due to an alternative theme I have installed for Blender)
After that, my render time for GPU 256/256 is 1min 35s, which is a lot more reasonable.
PC specs: i5 3.10GHz, NVidia GeForce GTX 1060 6GB.

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