Fortunately, W1zzard tested this scenario, and the conclusion is that using the graphics card at x8 4.0 is not going to cause any significant performance degradation, a very minor one (~2-3%) occurs at x8 3.0, so dropping two generations, at effectively just about 25% of the bandwidth that a 5.0 slot would offer with the same lane count.
The new NVIDIA GeForce RTX 5060 Ti uses the modern PCI-Express 5.0 bus interface, but supports only an 8-lane configuration. This is just half the bandwidth of the higher-end models, which run at x16. Are you in trouble with an older system, which only supports running at PCIe 4.0?
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It has little bearing even going all out with the 5090:
The new NVIDIA GeForce RTX 5090 is the first high-end graphics card that makes use of a PCI-Express 5.0 bus interface. Are you in trouble when trying to run it on PCIe 4.0? What about x8, like when an SSD is using up some bandwidth? We've also tested various PCI-Express 3.0, 2.0 and 1.1 configs...
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I understand you may be meaning WDDM shared memory spill (when 8 GB is exceeded), but regardless of the lane count available experience will take a hit and this is a scenario that should be avoided at all costs.
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