I'd consider static type checker as optional if you are playing around with small or throwaway scripts, but for any half serious project, it should be a requirement.
IME I don't consider it slow, and it's definitely not visibly slower than pylint, flake8, or similar tools for other languages.
Dropbox, which has one of the biggest Python codebases, uses it, and most mypy maintainers including Guido work there.
Guido hasn't worked at Dropbox since 2019. He retired for a bit, but is now in the Developer Division at Microsoft.
I see a lot of sort of 'programming culture' for given languages, practices, even discussion of working conditions and etc that on social media are taken as standard operating procedure. Highly active and even highly knowledgeable folks seem to portray various practices and etc are a given ... that I suspect are actually outliers.
What I would really be interested in, is do library developers use or intend to use mypy? Because if your libs are typed checked and your APIs are well constructed, a lot of that safety will flow to the clients.
Maybe those 40% would love to use it if something was changed or fixed. I think these numbers are frankly amazing, a year from now we reach an inflection point where mypy is used by the majority of some segment of the Python developer ecosystem.
In general, people who follow him are going to be more "into" Python than someone who just uses it to get their job done, and thus more likely to use optional tooling with a large overhead like mypy.
That will skew the results in favor of mypy in this poll. Though the poll getting on the HN front page will balance that somewhat.