Z80 I believe was something like 4 cycles per instruction and only a megahertz, so we would be talking at least 16x slower than a M4. You would need something between a 486 and Pentium to get close to the M4, and then evenn further to get to the M7. If I remember correctly you couldn't even decode MP3s in realtime until the faster 90+MHz 486.
There was speech synethsis on the apple iie in like 64k of RAM and a 1mhz CPU.
Speech synthesis is far far far easier than parsing a human voice, especially when it doesn’t need to sound realistic (as was the case back then).
In the old papers about problem, there was vocabulary size of 64K words, because nothing was working for bigger vocabularies.
It is done using SVD and improves speed without sacrificing memory access patterns.
It would decode mp3, up to 256Kbps, over NFS, 95 percent CPU utilization.
I was quite surprised.