You are facing >14h delay in email delivery. EmailLabs reports dozens of thousands of emails are sent in 1-2 minutes but Cisco Cloud Email Security Appliance (ESA) reports receiving emails from your farm many hours later.
There is a memory leak bug which affects every AsyncOS 13.5.1 build as it's caused by a config setting that is automatically enabled, and requires manually disabling. This is causing a slight delay in mail processing and SMTP responses of the ESA. This delay also adds to the total processing time, which results in “Total Time Limit for All Inbound Connections” being reached.
Since the bug is fixed, we recommend upgrading to a newer AsyncOS version. Refer to the Cisco site for more information about known fixed releases (you should have an active Cisco account).