Sometimes constituents will report that an autoresponder or other email was slow to be delivered. This typically happens with large email providers like Gmail, Yahoo, etc.
An email such as an autoresponder can be queued in the Luminate servers before it is sent to the email provider. We do this when necessary because some email providers have rate limits on how much we can send to them in a given period of time. Rate-limiting can affect any aspects of email - campaign email, constituent-generated email, autoresponders, anything.