Just to be sure that a problem is related to User Role Editor (URE) – is a page loading faster for the same lower admin access user if you deactivate URE?

Right, if you set additional restrictions via URE add-ons, URE adds filter conditions to the related SQL queries and it adds time to the page processing if you have a lot of posts and pages.

Generally, caching plugin like W3 Total Cache or similar can help. WordPress will used the result of SQL queries from the cache instead of sending them to the MySQL server again.

Also you can revise the restrictions conditions which you use in order query checks less conditions. You can test with different criteria: allow or block. For example if block criteria leads to checking much more pages quantity, switching to “allow” criteria may enhance productivity.