handling of non expanded JSON-LD

[IllDepence]

when a JSON-LD document is received for the first time its context has to be dereferenced -- this takes time

when the document is loaded from DB during the handling of a request at a later point in time, context is not dereferenced because this takes time. Ideally JSONkeeper should keep a dictionary of context URIs and their dereferenced contents (if possible) and use these for quicker local expansion.
→ can JSON-LD contexts be assumed to not change over time?