I´v seen similar behavior and have an Nvidia-card. I also thought the setting “prefer maximum performance” would mend that, but no. I used a gpu-monitoring software “GPU-Z” to see what was going on and it is indeed gpu-load that alters inconsistent but is not remedied by performance setting.
Findings:
Instant action - Feet wt: 125fps; GPU-load: 45%, CPU:21%
Instant action - Feet dry: 75fps; GPU-load: 30%, CPU:23%
EDIT: Tested FPS -quest 🙂 Focus shadow and water enviroment shadow stalling the system=Driver issue perhaps
VIDEO-Memory remains after first 3d to second 3D: 500-700mb and stacks= Dont do that.