One of the PDF files on the GitHub page has some info about the software implementation:

"* The original C version running on Z80 @ 4MHz was really slow! (maybe 25% of HP-25 speed)

* I started to re-write the emulator in Z80 assembly

* would love to finish this someday, but...

* Then I noticed that CMOS Z80 are available rated up to 20MHz

* I ended up running the Z80 at 16MHz, which results in an emulated speed within about 10% of the original HP-25. Not bad"

The other interesting thing is that this uses the 40-pin DIP version of the Z80, which will be discontinued on June 14, 2024: https://news.ycombinator.com/item?id=40083885