VERIFIED SOLUTION i
On the EngageOne Content Author server, a message was getting rendered in a few seconds but on a remote computer, using web "http://<SERVER NAME>/CCMContentAuthor", it rendered in 45 minutes.
This behaviour is expected because if one does not specify any image in a domain, images from all other domain are considerd a part of that domain and hence are transfered to the client machine.
In this specific case, very large images were not defined in the domain. Due to this, a large amount of data was transferred over the network, which was taking lot of time.
ResolutionUPDATED: September 15, 2017
Add a dummy image in the domain to render the message quickly.
Software Affected: EngageOne Content Author