R9 390 8GB: 4 monitors slow, 3 monitors fast

If I have a few office applications open, everything seems just fine with (4) 1440x2560 monitors running off my single R9 390. If I use Photoshop, even constraining Photoshop to a single monitor, or if I have an excessive amount of office applications open, my computer runs almost as if I would expect it to if it were using the CPU for graphics rendering rather than the video card. If I disable one of the 4 monitors so it's only using 3 for my desktop, it instantly runs at the speed I'd expect. Re-enable the 4th monitor, and it's back to being slow.

Am I running into some limit I wasn't aware of? I got this video card so I could run all (5) 1440x2560 monitors off a single card. (Haven't hooked up the 5th. This card has (3) DisplayPort, 1 HDMI, and 1 DVI-D.) I don't do any high end gaming, so didn't see the need to get 2 video cards, as this one looked like it could handle it just fine. I run the monitors vertically, so we're talking about that it works fast at 4320x2560, and slow at 5760x2560. Well within the card's specs from what I could tell.

Looking at GPU-Z, with 4 monitors and Photoshop open and being laggy, GPU-Z shows me memory usage of 760MB dedicated 199MB dynamic, 1-11% memory controller load, GPU load usually around 0-5% but once every 10 seconds or so spikes to 99% GPU Load even sitting idle (that part's weird), 64C temp with 29% fan speed.

Tuing one of the 4 off and Photoshop open and responsive, GPU-Z shows me memory usage of 703MB dedicated 224MB dynamic, 1-11% memory controller load, GPU load usually around 0-5% but once every 10 seconds or so spikes to 99% GPU Load even sitting idle, 64C temp with 29% fan speed.

So GPU-Z isn't really indicating a difference, and it's not showing anywhere near the 8GB of video memory is used up. A 4th monitor is only taking up an extra 53MB of memory or so, which is what I'd expect. And, again, I'm talking about a situation of using Photoshop on a single monitor, so it's not like the issue is Photoshop eating up a ton more video memory because its size is any different.

(3) Acer K272HUL bmiidp 27" 1440x2560 32-bit color @ 60 Hz * All verify through OSD as DisplayPort 1.2 * Running (1) DisplayPort to one of these, then chained to a second (Using DisplayPort 1.2 can run dual 1600x2560, so dual 1440x2560 is within spec.) * Running (1) DisplayPort to the third of these, will chain to a fourth soon (1) Asus PA272W 27" 1440x2560 32-bit color @ 60 Hz * DisplayPort 1.1, so gets it's own DisplayPort cable, can't chain

Asus Radeon R9 390 DirectX 12 STRIX-R9390-DC3OC-8GD5-GAMING 8GB 512-Bit GDDR5 PCI Express 3.0 MSI X99S SLI Plus motherboard Intel Core i7 5820K CPU 64GB DDR4-2133 Memory Driver 15.30.1025.1001-151204a-296872C Windows 7 Ultimate 64-bit

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