GTX Titan vs. sixfold GTX 750 Ti renderfarm

[Stefan]

Source: BlenderArtists

Someone (nickname Cycliste) on the French Blender Clan Forum made a quite interesting test using 6 (yes six) 750 Ti cards and comparing it to the more expensive Titan

Even if you don’t read French you might like to have a look at his rig and the test results.

I test the Cavalier scene in 2.69.11.
With 1 card i get 14:45.41
With 2 cards i get 7:34.43, so i beat a titan with only 2 of them.
The Time of 3:29 with 6x 750ti in 2.69.11 seems slow for me.

I wonder why the speed increase is not proportional to the number of GPUs.
The culprit could be either Blender, CUDA or the PCI-E extensions cables used in the renderfarm?