Forza Horizon 5 (1551360) - world runs in slow motion at steady 60fps, UI normal speed

System: CachyOS, Hyprland, Ryzen 9 5950X, RTX 3050 8GB, NVIDIA 615.71.09
Proton: proton-cachyos-11.0-20260703, also tested Proton Experimental

Symptom: 3D world and driving run in slow motion while the UI/menus run at normal speed.
FPS reads a steady 60, GPU 80-90%, VRAM 3.5/8 GB. Unplayable.

Ruled out: clocksource (tsc), CPU governor (performance), VRAM, refresh rate,
Proton Experimental, PROTON_ENABLE_NVAPI=1 + VKD3D_CONFIG=no_upload_hvv,single_queue,
PROTON_ENABLE_WAYLAND=1, gamescope, WINE_CPU_TOPOLOGY (8 and 16 cores),
verified files, moved to NVMe, fresh prefix.

Finding: PROTON_LOG shows ~2.19 million EXCEPTION_ILLEGAL_INSTRUCTION (c000001d)
in 107 seconds (~20,000/s), all from ~20 addresses inside ForzaHorizon5.exe,
caught by a vectored handler at 0x145715998 which returns CONTINUE_EXECUTION.
They occur in chains of ~12 per event (~1,700 events/s). Top addresses:
0x145EA273E, 0x145EA25A0, 0x145E9DCE0, 0x145E9DD3C (~182,950 each).
The on-disk .text at these addresses is encrypted, so the exceptions look intentional
(anti-tamper), and their cost under Wine is likely slowing the simulation thread.