RASM powerful Z80 assembler
Rasm was designed to be damn FAST and handle HUGE projects. On a modern machine, it can assemble millions of instructions in a blink of an eye. It's way faster than the best alternatives. Rasm also runs on almost any platform : Linux, MacOS Intel or ARM, Windows 32 bits or 64 bits, MS-DOS and even MorphOS
There is a cool French and English documentation, you should read it ;)
The project was born a few years ago (2017) because i needed a really fast assembler in order to finish my demo before the deadline of the Alchemy 2017. The project was quite huge with approx 350.000 words to assemble but thanks to rasm, the 512K cartridge compiled in a blink of an eye even on my slow Atom CPU. Now Rasm is the reference for huge projects and ultra-fast assembling.
BTW there is many other assemblers you could try: