Optimize rule search fields for slow request handling

[danieliser]

Per this users complaint we need to optimize for sites with slow hosting or large datasets:

• https://wordpress.org/support/topic/cant-use-content-is-a-selected-page-after-update/ ( slow to load )
• https://wordpress.org/support/topic/content-control-stopped-working/page/2/ - ( slow to load, but some items not found even direct search )

We have already changed to load as many as possible during initial query, and debouncing more requests. We have also cut number of initial requests in half.