The teams I’ve worked on with functional programming always had super senior engineers, people who had previously spent serious time on core GHC development and people who had worked as compiler engineers at e.g. Lightbend. They displayed all the same sources of slowness, defects, deadline and/or feature creep, bad designs leading to required refactors or rework, etc., as any teams I’ve been a part of that do everything in Python.
Static type enforcement is just a tool like anything else. It locates a certain class of code problems at compile or type checking time, that’s all. In most large projects, speeding up the detection of this class of errors is just not very important, and can be extremely well approximated with low effort test cases and other techniques.
This is a red flag to me. Of course the tool doesn't guarantee the result, but your story smells of incompetence. I've introduced FP and a focus on types-as-a-tool at the places I've worked and hired accordingly. Quality and speed of development have gone up after minimal time investment. A battle of n=1 I guess?