The usage of "lookup select" is a great improvement over the "standard select" in screens to choose the Custom Fields to create validators, conditions and post-functions, like bellow:
However, in large instalations, with thousands of customfields, the fact of all options in these selects are loaded during the page load time, the screen performance is poor.
Also, there are several screens in the JWT wich use more than one selectboxs.
A better approach could be load only the system fields during the page load time and another fields is REST aware.
How about that, Fidel ?