[nevcairiel]

The 40% is a comparison against H264 at 1080p, since that what majority of streaming is using right now, and what they hope to push AV1 into, since HEVC never took off in that space.
Higher resolutions would of course benefit more, but due to H264 still being prevalent there, it was not used often. Maybe that'll change when modern codecs take over?

Of course streaming is a special use-case for encoding wanting extra low latency, which precludes some features, and hardware encoding also tends to avoid some features that are not well suited to hardware processing. But 40% improvements for streaming should still be quite nice.