After a full investigation we are able to confirm this has not been an issue coming from Postmark. Instead this has been related to a network issue due to a router failure in Sweden and the Netherlands regions.
Customers experiencing timeouts had network traffic that was caught up in this, and the traffic never made it to Postmark. The timeout was network based which is why we were not able to see anything in our logs that pointed to a Postmark specific issue. Thank you for your patience while investigated all possible sources.