I spent many happy years making the Z80 dance, from Summer 1976 through Spring 1989, architecting and writing statistical multiplexors at MICOM Communications in LA. Micro 800 line. That is a Hard Real Time application, and the hardware for the first model was only 4KROM and 4KRAM. I made one Z80 do what the competition (CISCO) swore required 5, and laughed discreetly at us, but it worked, sold out the bottom end market from under them, and suddenly we had a runaway and I got an excellent team to buttress and ultimately take over the baton from me. I would dream in Z80 ASM in those days.

The Z80 had two hardware features that made it possible, and that nobody here has mentioned yet:

1: two sets of working registers, with a one byte instruction to toggle between them. I instruction cyclke to context switch between application and interrupt handlers. Only the TI 9900 came close with its in-RAM working register sets.

2: hardware vectored interrupt dispatch, fully suppported by theor accompanying peripheral line. Just a few clocks from raising an interrupt to executing the first line of code (typically switch context then ...) of not only the device but it's reason for interrupt code. No code wasted interrogating bitfields and scanning registers. 2 usecs interrupt response time. Faster than an Intel i960 with its 50MHz clock, that came along later.