Daniel McNab tracked down a missing barrier in the pathtag scan shader, which was causing artifacts on Vulkan (but, as these things go, not affecting mac). The same day, Max finally got around to installing the RTX 4080 (which was a Christmas present). That means I'm now able to run Vello and profile it in Nsight.

The results are jaw-dropping. Not just butter-smooth zoom and pan paris-30k at 120fps at 4k, but Nsight reports <1.4ms compute time to render.