The most costly in terms of rendering are transparent water and much detailed vegetation with high resolution textures, combined with very detailed terrain features, especially if they are also underwater. And lateral displacement, hence at least one compute terrain (or compute normal). And this in PT is slower than RT renders.
So I tend to keep the finest/heaviest veggies to the front (and not plant miles and miles of fine grass), and use lowres plants and trees for distance. Items such as far reed beds, or grass can easily be made with the internal grass (give it some color). Also I tend to not use the compute terrain, only as a (deadend) sideline to sit veggies on. Of course some compute terrain will be needed if you need precise lateral displacements based on slopes.
You can often reduce quality/size of textures too. 4K textures aren't needed for trees further than 100m away, for instance. Make them 1K or so.
I mostly render at detail 0.5 and AA6 (default), which I think is pretty good. Only specific renders/clients, whatever, need higher settings.
There's more to be done, but that needs a specific question.
