technically getting stutter from "main thread means your main game thread is too slow. DX11 ist the true bottleneck here.
DX11 loads CPU core really badly so every core after the fist oone just get less and less to do and the main thread need to handle the info for the GPU the Game engine the DX11 com and the 2nd just makes game and DX 11 and the rest only Game so its very likely to get limited by mainthread on olde CPUs
DX12 allows more cores to send info to the GPU and and more equally workload.
yes ik some update really broke the game and i needed to feel this way but DX11 could be the issue here mostly