Repeated string concatenation is slow on some python implementations, in particular PyPy

[Jacob Walls]

There is a ​known limitation in PyPy that repeated string concatenation is slow. The discussion there observes that CPython has an optimization, but it's fragile and best not relied on.

Seokchan Yoon (thanks for the report!) raised this with the security team, and we decided any hardening here could be done in public.

To close this ticket, we should:

• implement targeted optimizations in django.utils.numberformat.format() to avoid repeated string concatenation
• audit for other similar cases
• Add a disclaimer to our ​documentation around PyPy, for example, wording like:

That said, a lot of a web framework's work is done by concatenating strings, and PyPy has an issue with that (see ​https://pypy.org/posts/2023/01/string-concatenation-quadratic.html). This may cause performance issues, depending on your use.

[Natalia Bidart]

Thank you Jacob!

[Varun Kasyap Pentamaraju]

I am willing to contribute

[Varun Kasyap Pentamaraju]

Created PR ​https://github.com/django/django/pull/20045