I'd imagine that a modern kernel has a lot of pointless fat in it and that the 5-hours figure could be reduced if perhaps only slightly, with appropriate optimization effort. It might even be useful to deliberately have such a super-slow (but reliable, since it runs on highly deterministic microcontroller-class HW) benchmark in the mix, as you would be able to make very precise and fine-grained measurements about how even seemingly-unrelated changes might impact performance within the kernel, whether for the better or worse.