Antoine Pitrou asked me to document the performance and memory overhead of -X dev:

https://mail.python.org/pipermail/python-dev/2017-November/150578.html

For example, I measured an increase of +50% on the peak memory usage (measured by tracemalloc) when running test_os.

Antoine proposed:

"""Currently, developer mode adds negligible CPU time overhead, but can
increase memory consumption significantly if many small objects are
allocated.  This is subject to change in the future."""

https://mail.python.org/pipermail/python-dev/2017-November/150579.html