Found a copy of ZEXALL, assembled natively for the Z80 (ZEXALL.COM), the z80 instruction exercizer, one of the suite of z80 emulation test tools, but for CP/M. I'd run many of these early on, compilable C versions that ran in Teensy. This is the first time in most of year since I've run one.
On CP/M it runs about twice as fast as a reported overlocked 25 MHz z80 chip, consistent with my half-assed estimate of mine being 60 MHz equiv. This doesn't matter and I don't care; it's way over "fast enough", more than 10X any real machine end to end.
But under MP/M, wow.
ZEXALL.COM runs
CP/M: 19:30 (19 min 30 sec)
MP/M: one instance running: 23:27
MP/M: two copies running: 46:50
Now that's low overhead! This is with four consoles active and the margin clock running, and for about a minute or two, me poking at a console. Oh, that console was nearly unusably slow, but it worked fine. 20/40 minutes of solid CPU utilization, no I/O, is a good test of simple preemptive task switching.
MP/M with the modern four window display, USB, and SD cards is revealing the just fabulous and forgotten/neglected work DR did.
Real multitasking on the crappy z80, 13 years before Windows 98 brought same to the x86.