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.

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):

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

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

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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