What started as a problem with Skyrim, turned out to be a bigger problem with my system. Basically for some reason I am getting lower performance and stuttering with Crossfire on.
At first I thought it had something to do with Skyrim, however after checking I can say that I am getting lower performance with Crossfire in all games, together with some horrible stuttering.
I copied the following from a post I have on overclock.net so you can check it if you need some extra information regarding my system. Since I cannot post links, I'll copy everything here.
Here are my Unigine Heaven Benchmark test results.
Unigine Heaven Benchmark 4.0
Intel(R) Core(TM) i7-3770K CPU @ 3.50GHz (3499MHz) x4 <-- This has been OCed to 4.5GHz
AMD Radeon R9 200 Series 184.108.40.206 (4095MB) x2
2560x1440 8xAA fullscreen
Custom <-- This is everything maxed out.
So this is happening in every game. I am getting lower performance with Crossfire on, than with Crossfire off. I just did a clean format, swapped the position of the cards (I put the top one on the bottom and vice versa). turned on the Uber bios mode, and it's still the same.
According to GPU-Z my cards are running on PCI-E 3.0 x 16 @ x8 3.0 interfaces (which is due to my Asus P8Z77 Deluxe which can use only one card at x16, but if used in SLI/Crossfire, the two ports go to x8), however I do not believe this is the problem.
Not only am I experiencing lower performance and stuttering in all games that I've checked with Crossfire enabled, I am experiencing the graphic bugs with the ENBs in Skyrim - stuttering and white/extremely bright sky. I have tried turning Frame Pacing on or off without any effect.
The only thing different in GPU-Z is that one card has GDDR5 Elpida Memory Type, while the other one has Hynix, but if I understand correctly this shouldn't have this effect?
What other troubleshooting can I do? So far I've checked that the CPU temp is stable at 55-60 at max usage, my HDDs are fine, 1000W CoolerMaster CPU is more than enough, and I am out of ideas. If one of the cards was defective I should have gotten worse performance in single GPU mode after swapping them, however the same thing happens - single GPU performance > Crossfire. I am clearly out of ideas, except for maybe some problem with the motherboard, however it was bought about a year ago, and I didn't have a problem 1 month ago when I had 2xGTX 680s in the same slots as the R9 290Xs now.
Intel Core i7 3770k OCed to 4.4GHz Liquid Cooled(S1366, 2.80GHz)
2 x AMD R9 290X (PCI-Express 3.0, GDDR5, 4096 MB)
Corsair Vengeance 16GB DDR3-1600 Ram (PC3 12800, 8-8-8-24
2 x Samsung 840 RAID0 SSD
Windows 8.1 64 bit
Samsung Series 9 27" 1440p LS27B970DS
CoolerMaster Silent Pro M1000 1000W
CM Storm Trooper
Asus Xonar DX + ATH-M50