PCI Express is backward compatible. It will work on PCIE 2.0 with no performance loss. The card is only PCIE 3.0 compatible and does not uses most of his features.
I want to buy this graphics card but am afred it might not work.
Will It Or Will It Not ?
Virtually no difference in slot performance , your all set , will work.
https://www.pugetsystems.com/labs/articles/Impact-of-PCI-E-Speed-on-Gaming-Performance-518/
Impact of PCI-E Speed on Gaming Performance
PCI Express is backward compatible. It will work on PCIE 2.0 with no performance loss. The card is only PCIE 3.0 compatible and does not uses most of his features.