It's even much worse than that ;)

The fastest Z80 instructions are 4 clock cycles, at least 7 clock cycles if they also need to load or store an 8-bit value from/to memory, and at least 10 clock cycles for instructions that load or store a 16-bit value.

I did a little deep dive into Z80 timings a little while ago using the Z80 netlist simulation from visual6502.org:

https://floooh.github.io/2021/12/06/z80-instruction-timing.h...

But for BASIC programs those timings don't matter much, since BASIC on 8-bit home computers was easily 100x times slower than assembly code doing the same thing (the difference is probably less though when floating point math is involved, since this also needs to be 'software-emulated' in assembly).