Importing manifests can be a very time consuming operation. For large manifests this process can take from half an hour to mulitple hours. That is well outside of the reasonable period of time for a synchronous REST call. The REST endpoint for importing a manifest should be converted to an asynchronous endpoint.
Pull Request: https://github.com/candlepin/candlepin/pull/1263
Marking as closed per Candlepin procedures as a fix has been merged or it has been determined to not be an issue.