When to use wp_schedule_single_event for async / non-blocking processes?

…if you have low traffic on your site it may never run due to the way how WP Cron works

In your particular example it should always fire. Since save_post runs just before a browser redirect, the subsequent request back to the edit screen will fire the cron schedule.

…won’t that delay the page rendering the next time WP Cron is triggered by a page load?

No. If you check out spawn_cron(), you’ll see it fires a parallel request to process the schedule.

From the codex:

Send request to run cron through HTTP request that doesn’t halt page loading. Will not run more than once every 60 seconds.

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