Explain the problem, eg:

The reason that the completion suggester is so fast is that it is not a real search. Instead, the query is performed entirely in memory.  This means that it looks at all documents in the index.  It can't be combined with a traditional filter as this would have an impact on performance.

Instead, we can add predefined ``contexts'' to limit suggestions to a particular subset of documents based on a list of categories, the value of a separate field or a geolocation.