Does an x1 card in my x16(wired as x4) slot slow down my x16 slot?

(Dell Optiplex 7010 MiniTower)

I have the following slot configuration:
1. PCIe x16 (with GTX750 in it)
2. PCIe x1 (Unusable: It's covered by the GTX750)
3. PCI (of no use)
4. PCIe x16, wired as x4

If I put a PCIe 1x USB3.0 card into slot #4 above, will it slow down the video card in slot #1?

I've read that if the x16's share the lanes that the PCIe might downwardly negotiate to the slowest card. But it seems like only vague mentions.

If you had two identical full x16 slots running two video cards in crossfire mode, then in that case then your board would set them at 8x each. But that's not your case.

You have one full x16 slot and it is owned by your cpu, meaning it bypasses your Platform Controller Hub and its lanes are not shared with the other slots so you are good to go. That x16 slot wired as x4 does go through your PCH and does share lanes with the other remaining slots. See pic.

Check the Bus Interface field when using gpu-z, to see your status.

Download TechPowerUp GPU-Z | techPowerUp

WOW! HUGE help! Thanks!