What would cause cron jobs to fail? WordPress site unresponsive after migration

After a server migration, on a custom-configured server, a large WordPress site is becoming unresponsive once moderate traffic is pointed at it.

There seems to be no seemingly apparent reason for this, as server specs are very much sufficient. One clue is cron jobs are not running. Refreshing pages will not trigger them to run, and they report as “running now” or are scheduled for -40+ years ago (aka, to force them to run now)

However, this fixes it, in wp-config:
define(‘ALTERNATE_WP_CRON’, true);

But, why? I feel like this is a symptom of what is wrong with the migrated website, as it does not function at all once it receives traffic (20+ second loading, and/or unresponsive). CPU load/etc, is withing “normal-high” range, aka normal. It just, seemingly dies. Disabling/enabling cache and cloudflare doesn’t change anyhting. The original site, same settings, functions great.

What would cause cron jobs to fail and a server to not handle traffic after a migration to a freshly configured server?