Task Manager, by default, does not show Iray activity. If you go to the Performance tab, select the GPU graph on the left, and set one of the graphs to CUDA or Compute) you would see any Iray activity on the GPU. However, given the 100% CPU usage I suspect that the GPU is not being used - most likely as it has run out of memory. Note that in the last couple of builds of DS turning OptiX Prime off doesn't actually do anything, it is always used for Iray with non-RTX cards.