Hi Bill and thank you for your report.
I ran Site Health tool on clean WP install and figured this issue goes away when it’s disabled. Too bad it’s not too informative though as our plugin has nothing to do with WP’s REST API. But after some researching I figured that this is how Site Health tool reacts on plugins that are using PHP sessions.
There is a testing version of the plugin available which gets rid of PHP sessions – https://s-plugins.com/testing-version/ . Can you install and let me know if this fixes the issue?
Overall site performance also went from 78% to 87% when the plugin was deactivated.
This is something I’m not quite following. Are you referring to the percentage displayed by Site Health tool? It’s not related to performance. It just shows percentage of issues/passed tests. When you deactivate the plugin, one of the issues goes away and this makes the percentage increase.
If you’re referring to some other performance measurement tool, please elaborate.