How WP-Cron works, and why it misbehaves

WordPress does not use your server’s real clock-based scheduler by default. Instead, on each page load it checks whether any scheduled task is due and, if so, fires it. That has two failure modes:

• Low-traffic sites: if nobody visits, nothing fires. Emails, syncs and cleanups run late or never.
• High-traffic sites: many simultaneous visits can each try to run the same heavy task, causing overlapping runs, database contention and CPU spikes.

Cron-Sentry addresses both ends of that spectrum, and gives you visibility into what is actually happening.