Why does Baldur's Gate 3 feel stuttery even when the FPS counter looks fine?

Baldur's Gate 3 can show a decent FPS counter while still feeling weirdly choppy. The usual pattern is smooth movement for a bit, then short spikes when the camera turns or a busy area loads.

What is the cleanest way to test whether this is a shader, CPU, VRAM, or overlay problem? The setup points worth checking here are DX11 versus Vulkan, launcher bypass, mod order, and save compatibility. I’m looking for a fix order that does not start with reinstalling the whole game.