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Something in the range of 20% faster. Not a great deal but just notable.
How does the 390 compare to my GTX 780? Fancy trying freesync but will obviously need an AMD card.
What monitor are you looking at if you don't mind me asking?
I think those monitor has pretty high minimum Freesync range (something like starting from 45Hz), so it might not be the best monitor for trying Freesync, unless you can make sure your frame rate will not fall below that.