For messages that are critical or time-sensitive, such as a one-time passwords, leveraging TRANSACTIONAL message type in Amazon Pinpoint is indeed the recommendation.
An SMS message is delivered to mobile devices through third-party downstream aggregators, SMS suppliers, and mobile carriers. That is, when you use SMS with Amazon SNS, Amazon Pinpoint, or any AWS service that integrates with either Amazon SNS or Amazon Pinpoint.
It is worth noting here that, SMS architecture is distributed by design. It's a best practice to avoid making assumptions about the reliability and performance of SMS message transmission for your applications. For more information, see RFC 5724 from the Internet Engineering Task Force (IETF).
Having said that, in case you are facing unusual delays specifically with respect to OTP messages, please feel free to open a support case with AWS using the following link.
Is there a separate message type for OTP in Pinpoint. We want OTP messages to be delivered soon than transaction messages
Pinpoint SMS OTP: Amount of retriesasked 4 months ago
Does using a separate account for sending OTP messages give us a better OTP SMS delivery speed than sending from one account from which we send out transaction messages & OTP together
Pinpoint SMS message sent but not deliveredAccepted Answerasked a year ago
Pinpoint Showing otp message delivered but not receivedasked 7 months ago
Can't receive OTP SMS in Saudi Arabiaasked 5 months ago
Pinpoint Transactional SMS marked as SPAM in UKasked 6 months ago
Is OTP invalid after verify ?asked 4 months ago
Receiving CARRIER_BLOCKED errors on Pinpoint SMS Serviceasked a month ago
AWS Pinpoint is not working for some provider in Indonesiaasked 3 months ago