How Much Slower Would a PCI-E x16 2.0 GPU Run On A Normal PCI-E x16 Slot?

I'm only asking because ive learned all pci cards are backwards compatible and i'm looking for a graphics card but all of the good ones are PCI-E 16X 2.0 or 2.1 while i have just a PCI-E 16x slot who much performance would i lose if i were to use one of the higher cards on my motherboard?