Move Preload Cache to the background
I don't have a reliable way to split the preload cache process off to another thread other than wp-cron. Using something like pthreads or php-fpm both rely on the specific system being installed to. However, since this is a wordpress plugin, we know that wp-cron will always (at least to the extent that we know) be available to us. Therefore, the only reliable way to move processing of the preload cache off to an 'outside' process is wp-cron.
For the 'refresh now' links and methods, we can switch those to wp_schedule_single_event() actions: https://codex.wordpress.org/Function_Reference/wp_schedule_single_event