Thinking about it, I’m not sure if unsharp mask would use a GEGL filter that uses OpenCL, though you did see a difference in speed. There’s a list of which filters have and haven’t been ported here, though the website is down as I write:
https://wiki.gimp.org/wiki/Hacking:Porting_filters_to_GEGL#Port_of_new_GEGL_ops_to_OpenCL
Another factor might be that the Nvidia OpenCL implementation has been fairly weak, though I can’t see why it wouldn’t off some speed up.