Webhook handler latency regression

You receive payment webhooks from a vendor to mark orders paid. At 14:30 a fraction of orders sit in 'pending payment' though customers were charged. The vendor's status page is green and they confirm they're sending. Dashboards: your webhook endpoint is returning 200s and inbound webhook volume looks roughly normal, but your endpoint's p99 ACK latency crept from 80ms to 6s starting 14:25, and the vendor's delivery dashboard (shared) shows it backing off and DELAYING redeliveries because your slow 200s look like near-failures. No vendor outage; you deployed a webhook-handler change at 14:20. How do you triage and mitigate?