For the 3950x:
The PCIe lanes connect different parts of the system. Often when people say “it” has a certain amount of lanes they get a bit sloppy with what they mean. This video (at 3:00) explains how the lanes are allocated.
So as you can see in the video on the left there are 16x lanes that are typically used for the GPU. So you would put your 1660 video card into that slot and get maximum bandwidth for that card with no restrictions. There’s then two x4 NVME connections straight to the CPU (one more than I remembered). Then finally there are another 4 lanes and those go from the CPU to the x570 chipset itself.
The important point here is that the x570 chipset will actually connect to a bunch of other devices including other PCIe slots which of course then uses more PCIe lanes. That’s what you see in blue to the right of the image. The devices in those PCIe slots to the right go through the x4 lanes connecting the x570 chipset to the CPU. So that is a potential bottleneck for those lanes.
But you can also see in those blue boxes to the right that the motherboard manufacturer gets to choose how to use the resources and some of that can be SATA connections. There’s also the grey box where it says “4x SATA” which is another point where you have SATA connections.
So actually all of those SATA connections AND PCI slots go through the same x4. So that’s the bottleneck as far as lanes go.
On the Threadripper you still have 4 lanes as a bottleneck the same way, but they’re slower spec and so the bottleneck is actually worse. You would still connect your 1660 straight to the CPU so that’s fine, but it’s the resulting 6 HDDs that have to share that remaining bandwidth.
You should ask around and see what people say about “Huge Orchestral Libraries” and what drives they recommend that you use!
(My guess is that IF you end up using spinning HDDs then you have enough bandwidth, the drives themselves will be the bottleneck. And IF you choose SSDs instead then you’ll still most likely be fine unless you choose m.2 NVME drives (more expensive) and only then could you possibly maybe gain from using THreadripper, but just maybe.)
So again I really don’t see how this older version of Threadripper will give you anything you really benefit from unless I’m missing something. If you’re looking for an upgrade then maybe the latest generation is better (I don’t know) or you’ll be far better off spending money on NVME storage. And incidentally you can get a PCIe card that accepts NVME m.2 SSD drives on it. Obviously more expensive though.