Yeah, it is pretty good for a motherboard released in 2005 (AsRock 939Dual-Sata2). The other challenge was finding an AGP card which worked well in modern Linux. The winner was a GeForce 6800XT. The 6600GT was almost OK, but 1080p video playback was skipping frames, perhaps related to the bridge chip. HD4000 series all had issues with something in Linux, e.g. choppy scrolling though files in the file manager, or scrolling issues in a PDF viewer, etc.

I had also tried some LSI MegaRAID SATA 3.0 controller, particularly because it had XP drivers, but that card had all kinds of problems on this system.

You have quite the number of drives on your system. 306 MBytes/s in RAID5 with platters is pretty impressive, especially if you are at PCIe 1.0a. What motherboard is that in?

By way of comparison, on my screenshoot you can see 4.0 TB Drive called "Adaptec Storage". That contains two ST2000DX002-2DV164 drives in stripe. These are hybrid drives, 2 TB each, and I think with 8 GB of SSD cache. With the same 10 MB sample size, 100 samples, etc, they their average read rate is only 189 MBytes/s. So I am left wondering how you can pull off 306 MB/s.

Board is an Asus B85M-E, with i7 4790S. They're old ex-corporate stock that was getting liquidated on Ebay.

PCIe 3.0 x16 - GTX1050
PCIe 2.0 x16 (x4 electrical) - with the 3Ware 9650SE
PCIe 2.0 x1 - Magewell HDMI capture

Disks are 5x Seagate Exos 8TB - 7200rpm, 256MB cache. Individually using the 'disks' benchmark as before, they get ~165 to ~175MB/sec.

The 3Ware card has a 256MB cache, but I'm doing all the RAID in software using mdadm. I'm almost certain that the XOR performance on a 4th gen i7 is going to be better than a 12 year old controller.

Ahhh, you've got PCIe 2.0. Your benchmark results make more sense now. I think those drives should do 250 MBytes/s individually. Have you tested them on a SATA 3.0 card to see what the results are like?