It depends on the document ID you are using to retrieve a document. Alfresco CMIS document IDs look like this (note the ";v.v" part):
workspace://SpacesStore/xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx;v.v
Alfresco also accepts document IDs that look like node IDs (note the missing ";v.v" part) to retrieve documents, but always returns the format above.
workspace://SpacesStore/xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx
The OpenCMIS cache uses the first document ID format (with ";v.v" part) as cache key. If you are using the second format, you will never get a cache hit and the client has to ask the repository again for the document data.