The performance mode on series x has also better assets (textures, shadows, ...). But that should only be the case because the one x has more memory for the game than the ps4 pro. Because it is just a BC game right now, the game looks generally better on series x (now).
But the series x seems to have more problems with the performance. But, well, the engine is just not optimized. The Performance on the base-machines is in the lower 20fps territory so I really don't expect stable 60. A quality mode for PS5 (before the next-gen patch) would be optimal.
That can mean anything. As the game uses BC mode I doubt that the decompression hardware is used. But kraken can be (like many other compression-techs) very kind to CPUs or really stressful. It is only a question how good the data get's compressed. And I really don't know if the hardware solution of the PS5 can work with the highest compression-settings. I somehow doubt that. Normally hardware-compressions are mostly only compatible with the lowest settings, just because fixed function hardware is not dynamic enough to always get the best setting for the data.
Btw, according to
golem.de the game on Playstation needs >101GB. But they wonder why.