I think that Orthos is the one between the known stress tests which doesn't stress the CPU much. I can see like anyone else that when Orthos runs it loads my CPU Cores at 100%, indeed, but curiously, when I do a Orthos Blend test I can still start-up Steam, launch TF2 and play for hours non-stop with a minimum amount of very brief hitches here and there (especially during the first five minutes, after which all plays smoothly).
But if I do the same with Prime95 or OCCT I can barely launch Steam in the first place (it does launch, but it takes AGES).