If you're installing a PCIe card in the bottom x4 slot, then the third NVMe SSD may step down to x2. Feel free to try it and see what happens.
Some notes:
- One of my NVMe SSDs in one of my Hackintoshes is operating at x2 because of such a bandwidth sharing limitation.
- Have I even noticed the speed difference? NO
- Does it make any practical difference in my workloads? NO
- During sustained read/write, every NVMe SSD heats up and slows down (thermal throttling). Try running any disk speed test utility repeatedly in succession and see what happens to your speeds.
- Do people in general overreact to x2 link speeds? YES
- You can always buy a Thunderbolt 3 external enclosure for NVMe SSD in order to get close to x4 link speed if you really and truly need it.
Ok, I had some time extra so I tested a few use cases.
I populated slot 1 with my Samsung 960 EVO 500GB boot drive.
————First test————
In slot 2 and slot 3 I put 2 Samsung 970 EVO Plus 1 TB’s. The CNVi slot was still occupied with the WiFi 6 card. Plus my 10Gbit Network card was in the bottom PCIe slot
I tested the speed of slot 2 and slot 3. Slot 2 gave me the full x4 speed (around 3000 MB/s) and slot 3 gave me a x2 speed. (Also X2 speed info in Device Info)
————Second test————
In slot 2 and slot 3 I did again put the 2 Samsung 970 EVO Plus 1 TB’s. The CNVi slot was empty. Plus my 10Gbit Network card was in the bottom PCIe slot
I tested the speed of slot 2 and slot 3. Slot 2 gave me again the full x4 speed (around 3000 MB/s) and slot 3 still gave me a x2 speed.
About 1500MB/s (Also X2 speed info in Device Info)
————Third test————
In slot 2 and slot 3 I did again put the 2 Samsung 970 EVO Plus 1 TB’s. The CNVi and bottom PCIe slots were both empty.
I tested again the speed of slot 2 and slot 3. Slot 2 gave me again the full x4 speed (around 3000 MB/s) and slot 3 gave me also a x4 speed.
(X4 speed info in Device Info)
————Forth test————
In slot 2 and slot 3 I did again put the 2 Samsung 970 EVO Plus 1 TB’s. The CNVi and bottom PCIe slots were still both empty.
This time I tested the combined speed of slot 2 and slot 3 in a Mac software Raid 0. And I got a combined speed of still 3000MB/s.
————Fifth test————
In slot 2 and slot 3 where both empty. The CNVi and bottom PCIe slots were also still both empty.
I putted my 2 Samsung 970 EVO Plus 1TB NVMe’s on my Asrock X16 NVMe and the card in the second X16 (X8 speed) slot with bifurcation option on in the bios. (X8X4X4 mode). I did not change the software Raid 0 settings in Mac OS.
And the speedtest did gave me the combined X4 speeds.
About 5500MB/s (Both X4 speeds info in Device Info)
————End of tests————
————Conclusion————
Removing the CNVi card did not make any speed difference for me.
Removing the bottom PCIe slot gave me back my X4 speed for slot 3.
Testing the speed of slot 2 and 3 combined in a Mac software Raid 0 gave me the speed of X4 total. So slot 2 and 3 share a X4 bandwidth.
Testing the same Mac software Raid 0 on a PCIe card without using slot 2 and 3 gave me the correct speed.
I hope this helps people with any problems they maybe facing. Every speeds mentioned in this post are all read speeds.