Following on this: I managed to build the openSWR on a CPU only system (with 16 cores), and run some basic test. I have made large polydata model from MRHead (supersampled it by 0.5 with isotropic scaling).
When using the Ubuntu 22.04 provided mesa, full 3D view swinging performance of Slicer (latest preview version) was <1 fps. It never utilized over 200% of CPUs (so two cores).
With the gallium SWR exposed, performance was about 10fps, utilizing 1400-1500% of the CPU (so 14-15 cores).