README.md

$ time python loop_with_delay.py
index = 0
index = 1
index = 2

real    0m6.202s
user    0m0.111s
sys     0m0.039s

$ time python loop_with_async.py
index= 2
index= 1
index= 0
['', '', '']

real    0m2.500s
user    0m0.339s
sys     0m0.071s

loop_with_asyncio.py

#!/usr/bin/env python3

import asyncio

async def run_command(*args):
    """
    https://asyncio.readthedocs.io/en/latest/subprocess.html
    """
    # Create subprocess
    process = await asyncio.create_subprocess_exec(
        *args,
        # stdout must a pipe to be accessible as process.stdout
        stdout=asyncio.subprocess.PIPE)
    # Wait for the subprocess to finish
    stdout, stderr = await process.communicate()
    # Return stdout
    return stdout.decode().strip()

async def go_do_something(index: int) -> None:
    print('index=',index)
    res = await run_command('sleep','2')
    return res

def my_long_func(val: int) -> None:
    task_list = []
    for indx in range(val):
        task_list.append( go_do_something(indx) )
    loop = asyncio.get_event_loop()
    commands = asyncio.gather(*task_list)
    reslt = loop.run_until_complete(commands)
    print(reslt)
    loop.close()

my_long_func(3) # launch three tasks

loop_with_delay.py

#!/usr/bin/env python3

# https://docs.python.org/3/library/subprocess.html
import subprocess
from subprocess import PIPE  


def go_do_something(index: int) -> None:
    """
    This function takes a long time
    Nothing is returned
    Each instance is independent
    """
    process = subprocess.run(["sleep","2"],stdout=PIPE,stderr=PIPE,timeout=20)
    stdout = process.stdout.decode("utf-8")
    stderr = process.stderr.decode("utf-8")
    if "error" in stderr:
        print("error for "+str(index))
    return

def my_long_func(val: int) -> None:
    """
    This function contains a loop
    Each iteration of the loop calls a function
    Nothing is returned
    """
    for index in range(val):
        print("index = "+str(index))
        go_do_something(index)

my_long_func(3) # launch three tasks