I actually did that for a more realistic comparison.

Example for jo:

docker run --rm -it debian bash
apt update && apt install -y jo nano
nano bash-loop.sh && chmod +x bash-loop.sh

#!/bin/bash
for ((i=0;i<1000;i++));
do
jo -p name=JP object=$(jo fruit=Orange point=$(jo x=10 y=20) number=17) sunday=false
done

time ./bash-loop.sh >/dev/null

Example for Python 3:

docker run --rm -it debian bash
apt update && apt install -y python3 nano
nano python-loop.py

import json
for i in range(1000):
print(json.dumps({"name": "JP", "object": {"fruit": "Orange", "point": {"x": 10, "y": 20}, "number": 17}, "sunday": False}))

time python3 python-loop.py >/dev/null

Versions:

Debian GNU/Linux 11 (bullseye)
jo 1.3
Python 3.9.2

Results for jo:

real 0m2.230s
user 0m1.106s
sys 0m1.076s

Results for Python 3:

real 0m0.027s
user 0m0.021s
sys 0m0.005s

So it seems like you're probably right about how individual invocations scale for larger amounts of invocations in non-trivial cases!

Note: jo seems to pretty print because of the "-p" parameter, which is not the case with Python, might not be a 1:1 comparison in this case. Would be better to remove it. Though when i did that, the performance improvement was maybe 1%, not significant.

Admittedly, it would be nice to test with actually random data to make sure that nothing gets optimized away, such as just replacing one of the numbers in JSON with a random value, say, the UNIX timestamp. But then you'd have to prepare all of the data beforehand (to avoid differences due to using Python to get those timestamps, or one of the GNU tools), or time the execution separately however you wish.

Edit to explain my rationale: Why bother doing this? Because i disagree with the sibling comment:

> The claim was that the Python snippet would be quicker than the jo snippet.

In my eyes that's almost meaningless, since in practice when you'll actually care about the runtimes will be when working with larger amounts of data, or alternatively really large files. Therefore this should be tested, not just the startup times, which become irrelevant in most real world programs, except for cases when you'd make a separate invocation per request, which you sometimes shouldn't do.

Edit #2: here's a lazy edit that uses the UNIX time and makes the data more dynamic, ignoring the overhead to retrieve this value, to get a ballpark figure.

Use time value for jo:

jo -p name=JP object=$(jo fruit=Orange point=$(jo x=10 y=20) number=$(date +%s)) sunday=false

Use time value for Python 3:

import time
...
print(json.dumps({"name": "JP", "object": {"fruit": "Orange", "point": {"x": 10, "y": 20}, "number": int(time.time())}, "sunday": False}))

Results for jo:

real 0m2.794s
user 0m1.422s
sys 0m1.313s

Results for Python 3:

real 0m0.027s
user 0m0.020s
sys 0m0.006s

Seems like nothing changed much.

Edit #3: probably should have started with a test to verify whether the initially observed performance differences (Python being slower due to startup time) were also present.

Single iteration results for jo:

real 0m0.003s
user 0m0.000s
sys 0m0.002s

Single iteration results for Python 3:

real 0m0.022s
user 0m0.017s
sys 0m0.004s

Seems to also more or less match those results.