What kind of Performance Drop would I get with a PCI Express 3.0 Card on a PCI Express 2.0 Slot?

AustinMxr1

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Dec 26, 2014
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I have an R9 270X which is PCI Express 3.0 x16 and it's on my motherboard which only has PCI Express 2.0 x16. What kind of performance am I missing out on? And also, I'm soon upgrading to probably an ASUS Geforce GTX 970 STRIX, which is also PCI Express 3.0. Would I lose more performance than the 270X because the 970 is a newer card? I heard something like that on here before. Thanks!