Thunderbolt 4 AIC + 2 GPUs

I'd guess LiveMixer is the better choice.

The first two PCIe slots are connected directly to the CPU (one x16 and one x4).
My guess is that the Thunderbolt card requires direct PCIe connection to the CPU, based on the explicit mention in the user manual. I don't know anything about the Thunderbolt expansion card, though.

Third PCIe slot is connected to the chipset, which means any interaction to the CPU will share the interface with all other chipset functions. Note chipset is connected to the CPU through a x4 link, and so all chipset functions that need to talk to the CPU (USB, SATA, etc.) will go through this shared x4 link.

It will add a slight latency also, since it needs go through the chipset.

Depending on what you're trying to use the GPU for, it might matter, or it might not. The fact that the whole interface is just x4 will matter more, again depending on workload. If you need to move a lot of data through the link, then the GPU will be bottlenecked. But it's still way better than using an x1 link.

I don't think the additional latency will matter much for most GPU workloads, so I don't think it'd matter that much if you use the 2nd PCIe slot (direct to CPU) or 3rd PCIe slot (through chipset).

But if the thunderbolt card can use the 3rd slot, you can maybe experiment with it.

Ryzen 7000-series CPU has total of 28 PCIe lanes, 4 of which will connect to the chipset, leaving 24 (edit fixed bad math LOL) lanes for user use. The usual distribution is:
x16 for the GPU slot.
x4 for the fastest m.2 NVMe slot.
And then the rest x4 are either split (e.g. PG Lightning) or given its own slot (e.g. LiveMixer).

I do wonder why nobody makes a board with x8 slots instead, like:
x8
x8
x4 (m.2 NVMe)
x4


Edited by eccential - 27 Jun 2023 at 3:33am

I just found out Asrock X670E Taichi does have two x16 slots that can be configured as x16/x0 or x8/x8 (directly to CPU). It does NOT have any more PCIe slots, but it has built-in USB4. I'm assuming it's using some Intel chip for the USB4 functionality, but I'm not sure. It says it's 40Gb/s capable, which matches Thunderbolt 4 AIC. It does say that DisplayPort Alt Mode through the USB-C connector will only work for iGPU.

I've no idea what your use goal is, but maybe X670E Taichi, with built-in USB4, is an option for you. This way, you can get a more balanced x8/x8 lanes to two GPUs, directly from the CPU.