External GPU and PCI lanes

[nawabkhan_u]

The bandwidth of a PCI 3.0 x16 is 15760 MB/s.the bandwidth of PCI 3.0 x8 is 7880 MB/s.
While testing graphics card on both PCI 3.0 x16 and PCI 3.0 x8 gives identically similar performance with just 1-2 fps difference.
How it is possible?
Similarly using external GPU with laptop which uses PCI 3.0 x4 lane gives performance as that of desktop graphics card with just only 10% difference.
Can anyone explain both this detaily?

 

[Guest]

It's rather normal. The pcie isn't even close to saturated.

Basically, that bus isn't the bottleneck. Between pcie2x16 and pcie3x16, you won't get much a difference. Same thing with pcie3x16 and pciex8. At pcie3x4, you might start to see a difference, and 10% sounds about right.

The bottleneck isn't between the GPU and motherboard, it's on the GPU. Or elsewhere.

 

[nawabkhan_u]

I don't understand.can you explain detaily.my question is how same graphics card give similar performance in PCI 3.0 x16 and PCI 3.0 x4 both have different bandwidth?

 

[nawabkhan_u]

My second question is about egpu.not about desktop class graphics card

 

[ThatVietGuy]

MERGED QUESTION
Question from nawabkhan_u : "External graphics card"

Latest egpu technology is by the use of thunderbolt 3 port.It uses PCI 3.0 x4 slot which provide just 3.9 GB/s.
But it's performance drop is just 10% while comparing with the desktop pc graphics card which provide PCI x16 lane with 15.7 GB/s.
Does bandwidth of PCIe slot doesn't affect the graphics card performance?

Repost - http://www.tomshardware.com/answers/id-3527445/external-gpu-pci-lanes.html