The host<>device interface (the pci-e slot) isn’t used during rendering.
Only before and after.
A single PCIe 2.0 lane supports 250-500 MB/s.
A PCIe 2.0 x16 thus could have a maximum of 7.8 GB/s.
If it runs with x4 you could shove a max of 1,95 GB/s in the VRAM.
So, assuming you got an 1.5GB card, filling the VRAM takes either 769ms or 192ms which I’d call neglectable when rendering frames that take longer than 1s to render
During rendering, the PCIe interface isn’t used anymore, all is done on-device that’s why it’s that fast. For the GTX 580, the on device speed, or memory bandwidth as it’s called would be ~192 GB/s.
Once the render is done, the data, most likely a 100mb-ish image has to be sent back from the device to the host. neglectable again.
The PCIe lanes only matter for stuff where you need to constantly stream data, like games for instance, where you need to fill geometry and textures from the ram to the vram.