I accidentally did a test with my i7-4790k + GTX 980Ti + WinXP x64 host.
The CPU's PCIe3.0 bus was running at only 4x, so its speed was like a PCIe1 x16.
I've experienced the following:
1. The GPU usage & temperature was slightly less then normal (so it run unnoticed for a couple of days)
2. The GPU & memory clocks were normal (1404MHz & 3505MHz)
3. The workunits' runtime went up by 123% (yes, more the doubled: 6h5m -> 14h14m)

These results confirm that old MB & CPU should not be used with high-end GPUs to avoid such frustration by similar workunits in the future.
(Also, the performance decrease caused by WDDM is higher than usual on these workunits)

I had a few WU take much longer than average a few days ago, with no system changes. Got a regular amount of cobblestones, so I assume the length was unintentional. Back to normal now.

Would you mind offering a few more details:

How old was the motherboard running the 4790k?

In my estimation, the 4790k isn't a very old CPU; what sort of motherboard were you plugging it into?

I accidentally did a test with my i7-4790k + GTX 980Ti + WinXP x64 host.
The CPU's PCIe3.0 bus was running at only 4x, so its speed was like a PCIe1 x16.
I've experienced the following:
1. The GPU usage & temperature was slightly less then normal (so it run unnoticed for a couple of days)
2. The GPU & memory clocks were normal (1404MHz & 3505MHz)
3. The workunits' runtime went up by 123% (yes, more the doubled: 6h5m -> 14h14m)

These results confirm that old MB & CPU should not be used with high-end GPUs to avoid such frustration by similar workunits in the future.
(Also, the performance decrease caused by WDDM is higher than usual on these workunits)

How old was the motherboard running the 4790k?
In my estimation, the 4790k isn't a very old CPU; what sort of motherboard were you plugging it into?

Even having several of these slow WUs in my average, my average completion time is currently in 2nd place

These results confirm that old MB & CPU should not be used with high-end GPUs to avoid such frustration by similar workunits in the future.
(Also, the performance decrease caused by WDDM is higher than usual on these workunits)

Yes and no. Yes, it is frustrating. But why shouldn't high end cards be used in older computer?

I am getting run times on the other GERARD WUs that are comparable to my new windows 10 computer.

This brings up another question about future WUs. Should they be more or less depend on CPUs?

Even fast computer pay a time penalty with the A2AR WUs, though it is much less than the older CPUs. If we want to have more efficient (faster) crunching, the WUs should be made less CPU depend, where possibly.

(Then we should use Double Precision enabled GPUs, which are very very expensive.)

If the Work Units are truly CPU limited on a 4790K, then you should see an increase in performance if you disable hyperthreading in the bios. The app would then have access to one full core rather than a single logical core.

If the Work Units are truly CPU limited on a 4790K, then you should see an increase in performance if you disable hyperthreading in the bios. The app would then have access to one full core rather than a single logical core.

Actually there's a complete thread for a similar batch in the news topic, started by Gerard himself :)
There is an explanation for the higher CPU usage of this batch:

I forgot to note that due to the nature of these simulations, some small forces have to be added externally and, unfortunatenly, these have to be calculated using cpu instead of gpu. Therefore you may notice some amount of cpu usage that in my case never surpassed a 10%.

Good stuff, This is the sort of thing LinusTechTips over on Youtube does once in a while, how does it fare at 8x?
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My computer recently downloaded a work-unit than normally take 9-10 hours on my GTX 970, and to my astonishment BOINC tells me that it will finish in 1d 03:16:51. It is the new chalcone229x2-GERARD_CXCL12_DCKCHALK. Is anyone else seeing this?

Have we entered an era where x8/x8 isn't fast enough any more?

This brings up another question about future WUs. Should they be more or less depend on CPUs?

Now that's the million dollar question.