Email notifications arrive 20 minutes late

Notification emails are arriving about 20 minutes after the reply is posted. Everything else is fast. Where should I look?

Check the job queue in Admin › Jobs. If the default queue has a backlog, something is hogging the workers.

Usually it's the image optimisation jobs after a big upload. Raise the worker count:

env:
  THREADWELL_WORKERS: 4

And rebuild. Mail goes in its own queue, but it shares the workers.

We had 4,000 queued image jobs from an import. Raised workers, queue drained in an hour, emails are instant again.