The mod for RDR1 improves the textures of almost everything in the game (with the exception of landscape textures and some other elements). The author has increased the resolution of all textures from the mapres.rpf file, except for landscape textures, since there is already a separate mod for this. NPCs, animals and other elements have not been improved since they are not in mapres.rpf.
This mod should not significantly affect performance. In addition, you can install other mods on top of it.
Installation: