Widget Visibility: audit for performance

As part of our ongoing performance work, let's see if we can improve the performance of the Widget Visibility feature, particularly the calls to WordPress.com.

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Reference:

> We primarily use Jetpack in offline/dev mode, mainly for the widget visibility options and the blocks, yet, it has been the most time-consuming component of our large multisite according to our New Relic APM. And it is the one with the highest external web calls (to [public-api.wordpress.com](http://public-api.wordpress.com/)) despite being in offline/dev mode.