This is by design. When you enable ECC you will have less graphics memory available . Please click on the green ? on ECC page in the Catalyst Control Center to read about ECC.
Yes, I know that the available memory is reduced, but not too much (less than 1 GB), the memory bandwidth is also reduced (about 23% less with ECC, according to AIDA 64 GPGPU benchmark).
What I'm trying to say is that applications runs much slower with ECC, even those, which do not require 7-8 GB of VRAM or high memory bandwidth,
If you open my attached images, in the LuxMark HDR test, I got 13388 points without ECC (rays/second 29951K), which is normal for a HAWAII GPU, but with ECC, it is only 1610 (rays/second 3623K), Almost ten times slower.
RatGPU finished under 149 seconds, with ECC it was 738 seconds.
And I know, this is not a gaming card**, but if you see the 6 years old Stalker CoP benchmark results, the performance difference is also very huge, it's not just about GPGPU, it affects a lot of applications,
Without ECC, 3ds max is butter smooth with 10+ million polys, sometimes even parallel with Vray RT activeshade, depending on the scene, but when I enable ECC, I experience slowdown from about 1.5-2 million polygons, and this is without activeshade, just in the viewport.
Currently the only program what I'm tested and don't saw any performance difference was the old SpecViewPerf 11 synthetic (OpenGL based?) test.
(**Rarely I'm doing gaming too, and the card handles even GTA V without any performance or image quality issue with high- very high settings at 1080p.)
I just want to know that is this by design too, or if isn't, then maybe a driver/OS or a hardware issue with my card?