Best practical method to compare the speed some PL interpreters

Depends how many different tasks you want to compare, and how many different interpreters you want to compare, and what you want to do with the comparison.

Find a comparison someone else already made, for example:

https://benchmarksgame-team.pages.debian.net/benchmarksgame/

Make your own time measurements:

https://github.com/tobami/codespeed/

https://github.com/sharkdp/hyperfine

See debian's benchmarkgame.

You need some cross-language apps/functions, adopt community improvements and publish the results.

Rosettacode also has a lot.