At 4K with RTX On, the frame rate plummeted to around 12 FPS. A system utility revealed that the GPU utilization was below 50%, as the rest of the Pi system bottlenecked it so comprehensively.

The poor CPU is getting hammered like there is no tomorrow.

It’s impressive that anything could run in 4K with the GPU connected by a single lane of PCIe 2.0 and the CPU translating X86 code to ARM. Maybe a future Raspberry Pi version will actually have enough power and bandwidth to make it somewhat practical.