What is the best way to test my speed between my computer and a Mikrotik device?
AFIK iperf isn't working.

Windows? Linux? MacOS?

For secure one thing: the speed must be trough the Router, not from PC to Router,
because one thing are the CPU, used for test, one thing is the "switch chip" if the test is with router between two devices.

I can test from macOS, Windows and FreeBSD

I can test from macOS, Windows and FreeBSD

on windows there is one app called btest, downloadable on mikrotik site,
https://mt.lv/btest
but the important part are what I have writed before

Copy that, thank you!