Lightweight Case Creation via Web Service

Our applications hosts a Web Service which creates cases.
Our service can be consumed by External systems.
Accordingly using the request data received, we create cases and return the case id or error parameters as response of the service.

One of the External System has a huge payload of Request Data -

1) Huge Payload of request data and attachment content.

2) After parsing, some data objects needs enrichment which calls subsequent interfaces.

As per present implementation - xml request data parsing and enrichment happens synchronously,
ie. full request content parsing and enrichment happens before a response is returned to the calling application.
Therefore: we often have a timeout issue before we can return a valid case id. (the handshake expires the 30 sec window.)

What can be the possible and best design approach to make this process asynchronous
(return the case id as soon as possible, parse and enrich later) ?