Does SES have cold starts?

To help prevent fraud and abuse, and to help protect your reputation as a sender, we apply certain restrictions to new Amazon SES accounts.

We place all new accounts in the Amazon SES sandbox. The sandbox status for your account is unique per each AWS Region. While your account is in the sandbox, you can use all of the features of Amazon SES. However, when your account is in the sandbox, we apply the following restrictions to your account:

You can only send mail to verified email addresses and domains, or to the Amazon SES mailbox simulator.

You can send a maximum of 200 messages per 24-hour period.

You can send a maximum of 1 message per second.

This is link to request production access - https://docs.aws.amazon.com/ses/latest/dg/request-production-access.html

Check if the Email receiving has been setup correctly (security setup is correct) and metrics enabled - https://docs.aws.amazon.com/ses/latest/dg/receiving-email.html

Also if you don't plan to send large volumes of email on a regular and predictable basis, we recommend that you use shared IP addresses and not dedicated IP addresses(Standard). If you want to use dedicated IP addresses in situations where your sending patterns are highly irregular, using Dedicated IPs (managed) is the better option.

You can monitor deliveries, bounces, and "delivery delay" events with event publishing to determine why messages are being delayed or bounced by the recipient mail server - https://docs.aws.amazon.com/ses/latest/dg/monitor-using-event-publishing.html