386sx 16Mhz

initial tss switch:
386 12345 switches/sec

software switching
using pusha:
386 19531 switches/sec

using less pushes:
386 20569 switches/sec

using O2:
386 22355 switches/sec

using fastcall for inner routine:
386 25388 switches/sec

tested on AMD kaveri, 3.9Ghz:
approx 40mil/sec software, 1mil/sec hardware

skylake 4.2Ghz in qemu/kvm:
45mil/sec software, 121k hardware (may be vmexiting)