Depends on the asset.
If the issue is present at the branch, and the tree is sort of similar to speed tree, then try to see if enabling early Z pass makes any difference.
Pcg and anytting epic has made in the past 3 to 5 years is just not meant for games (the ironiny of them having a Game engine is not lost on anyone, believe me).
Foliage needs to be put into the foliage actor stuff, because its internal code - used to, you should verify this to still be the case - partitions instances and HLOD gropus for game performance, unlike HISM or other instanced options.
Assuming that PCG is capable of outputting to the foliage, then the reasons for poor performance become the usual suspects - ei: this engine being made by people who dgaf about performance.
You can try swapping to forward shading to gain some marginal amount of fps while loosing a lot in the “looks” department.
Or you can try to optimize things to gain maybe 5 FPS after a month full of work…
Or you can change to a better engine (assuming you have the rights to the models and things you used) and actually make a playable game.
I guess the 3rd option here is to stick to engine versions that somewhat worked:
4.18 was decent. 4.25 was good - though worse perfeomance wise than .24.
4.26/27 lost 20% FPS for no reason at all that epic ever admitted to (goes back to the fact they dgaf).
Anything after the Chaos change is trash that can’t really be used to publish any real game that attempts to use physics, this does include any .24 build with chaos.
I guess you have a 4th option too? get a 4090. If you have one you could get another one… oh wait… Unreal engine can’t use 2 gfx because that would be TOO easy
Another thing to keep in mind is that more engine windows are open more FPS you loose as your machine overheats in agony to attempt to render things - even with a 4090 or better.
Epics idea of rendering anything stops at 1080p - so if you are set up for anything above that, everything is just your fault and the engine is not made by incompetent people
Because of this, you need to test/benchmark your final release with the engine closed (publish package and run it) so that nothing except your game exe sucks up those MS…