Over time, bake performance decreases drastically when using high sample count (500K or 1M).
The first 5 minutes of the bake are fine, and then, the GPU occupancy decrease over time.
It seems that the Editor his somehow taking breaks between each call to the lighting baking backend, making the GPU occupancy really low and bakes like it will never end.

This is not a duplicate of 1327130. In 1327130 what happens is that we are doing too much Convergence jobs and we get out of the Render() function of the RadeonRaysBakeLightmapTechnique after doing not enough samples. This bug, on the contrary, happens more on the editor side, were at some point in time, we start calling less often Render(). The Editor seems to make some weird pauses between each call to render and the overall GPU utilization drops. Whereas in 1327130 the GPU utilization stays quite high.