I have an AMD R9 290 4GB, and an AMD FX-8350, with 8GB RAM. When I'm flying in supercruise, the game gets steadily laggier and laggier, gradually going from a smooth 60fps to something absolutely unplayable until I drop out of supercruise.
This is a known issue that AMD have been *ahem* working on for a while now.
David said in the stream yesterday they think it was fixed and are just waiting for AMD to update their drivers. SO that will be sometime in the 2020's I would imagine.