bottleneck performance?

[Judge_Xero]

Typically if you have 2 X PCIe full slots then either will run in 16x or 8X (whichever they are designed for, as most Intel Boards are moving to 8X PCIe 3.0 where as AMD Boards still have Native 16x with PCIe 3.0)

But what typically happens is this.

Slot 1 16x - No GPU
Slot 2 16x - No GPU

You can put a GPU in either slot and have it work in 16x PCIe
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Slot 1 8x - GPU Present
Slot 2 8x - GPU Present

With both slots populated it shifts both to 8x - where as individually they are capable of 16x

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The only native 4x PCIe lanes are those little short ones that you would plug a wireless/SSD/Video Card/Network card into (which can also be Native PCIe 1x on budge motherboards)

The motherboard does not need to be Crossfire/SLI capable to have each of the individual slots to run in 8X or 16x, it just matters how many you have populated in how it divides the available communication lanes.