Its equivalent to x264 medium when using "quality" or "low latency quality. "High quality" just enables two pass on top of quality, which doesn't get it out of medium, but improves the bit distribution similarly to how two pass in x264 does.

All the gens of GCN through RDNA1 are on the spectrum of veryfast to fast, Pascal is fast; RDNA2 (due to inclusion in PS5 and XB2) meets Turing's new bar for quality (so no more low quality console streams on Twitch).

So, none of them compare to veryslow. Ampere will probably have a better encoder than Turing, but I think the fair comparison will still be against medium, not slow. The big change, afiact, will be work on h265, h264 for them is pretty much as good as they need.