I've only got a 1080p monitor connected, so can't try anything higher res. But it's about 20% cpu usage for 1080p @ 60fps.

Just to report:

Q9550 w/ GTX980 is very smooth. Can stream youtube 1080p w/ no issues. Can even run some games I like @ 1080p @ 60Hz @ medium settings. Using Win 10 Pro w/ latest Nvidia drivers.

I'm not sure how good the motherboard is. It's a Q35 chipset, which I don't think is ideal. Q45 would be better probably, but it's hard to find the exact differences between these old chipsets now....

Very interesting. I will assemble a Q9550 system soon with a GTX980. I'm interested to see if it encounters any of the same issues.

Did OP ever figure out the exact issue?