Thank you very much for what you try do do. But, if the site works now with free version of User Submitted Posts, why would I install something additional ?
It is obvious that the plugin has a problem.
Can you please install the “Query Monitor” and send me the total report. Query Monitor is a popular plugin for view debugging and performance information on database queries, hooks, conditionals, HTTP requests, redirects which will help us to get the actual report of the slowing issue.
First uninstall WP User Frontend Pro and test with the Query Monitor plugin and take the report.
Again, install the WP User Frontend Pro and test with the Query Monitor plugin and take the report.