I downloaded two of them in the different versions to have a look and test them.
The hevc encodes are often pretty good on this site. Not great, but passable. The same goes for the two videos I downloaded. The AV1 encodes here are worse. (Not to mention the horribly noise-reduced awful glowing encodes on recent AV1 releases from SLR Originals, but I digress.)
Generally, AV1 tries to remove all skin detail, which it sees as noise, and since porn is all about skin you have to work around that. That is not being done here.
First of all, don't encode on a GPU. It will be faster, but the result will be poor unless the bitrate is ridiculously high.
Rather, encode on the CPU. Use SVT-AV1-PSY 3.0.2, or whatever is newest. It comes with Staxrip. You can just replace the codec build in the encoders folder if you want to use another build.
Do not tune for SSIM or PSNR, they do not preserve skin detail. Outlines matter most for those. Tune for VQ.
Do not use quantisation [quantization] matrices. They will blur skin and only save a small bit of size.
You need to test-encode to see which bitrate (crf) is appropriate.
Change the bit depth to 10 bits [ConvertBits(10) in AviSynth], before encoding. The codecs are most efficient when using 10 bits. (It's not the same as HDR, because it doesn't change the colorspace, and all players that I know of play it fine.)
For the video: For Women: Royal Treatment
The hevc version is okayish, but the av1 version looks really bad.
Using the following non-default parameters to re-encode the large "original file" with SVT-AV1-PSY 3.0.2 I get a picture quality that is much better (a LOT better than the av1) than either of the two and a smaller file size:
--rc 0 --crf 33 --preset 6 --tune 0 --tf-strength 3 --enable-qm 0 --sharpness 2 --enable-overlays 1 --qp-scale-compress-strength 2 --psy-rd 3 --hbd-mds 1
For: Pink Me
She's a bit of a porcelain doll, and the background is monotone so the video compresses well with any codec.
Both the hevc and the av1 blur the skin however.
You get better detail and the same file size with the "original" recoded with:
--rc 0 --crf 23 --preset 6 --tune 0 --tf-strength 3 --enable-qm 0 --sharpness 2 --enable-overlays 1 --qp-scale-compress-strength 2 --psy-rd 3 --hbd-mds 1
These encode at about 7 fps on a modern CPU, depending on what you have. Skip the two last parameters to speed it up a few fps, but it affects the quality slightly for the worse. By all means use preset 4 or even 2 if you have aeons or a supercomputer at your disposal. You might get a slightly smaller file size.
Again: Do not encode on a GPU.