I had a suspicion, which resulted in what seems like a breakthrough.
My suspicion is that the abstracted network bits used for a container’s network stack might be killing the performance, since all other things seem equal (python version, etc.).
I did a test with adding this container option to the execution environment block in the ansible-navigator.yml configuration file.
container-options:
- "--network=host"
With that change, the performance now matches ansible-navigator inventory without the execution environment. Of course keeping this configuration presents a question about defeating an inherent security feature of the execution environment container, but at least now I know the general reason behind the performance issue.
Thanks to all of your suggestions!