I had a similar issue I posted recently here: Performance took a nose dive after duplicating the metahuman guard - Unreal Courses / Talk - GameDev.tv
I’m also using an RTX 3090 and AMD Ryzen 9 5900x with 64gb DDR4 memory.
I had to force the LOD on all the metahumans to ‘1’ and I got back most, but not all of my performance.
I don’t know the exact problem, but it’s definitely related to the metahumans.