Bug SceneManager.LoadSceneAsync lag spike on additive scene load / HDRP, unity 2021.2

I searched around and it seems to have been an issue forever. Unless this is a regression?
or maybe it had to do with terrains in the scene?
https://forum.unity.com/threads/scenemanager-loadsceneasync-additive-lag-spike.494874/
Has it always been like this? any workarounds, need this to stream open world/large projects

Loading 1 fairly small scene, includes 2 small terrains (300x300) next to each other, with some trees and grass.
All in all, 4 tree types, and 1 grass, plus a single local volumetric fog object.

There's quite a big spike, profiling I noticed the majority of the spike is caused by "QueuePrepareIntergrateMainThreadObjects" during loading. It's a 2-3 frame +13ms spike.
Since it has to do with culling for some reason, makes me think it has to do with terrains, maybe?



using HDRP, unity 2021.2.4
profiler data taken from a build