Is it possible to use an index for x-cache joins with linear index update time and query?

Index update cost is not linear but LogN: approximates to a constant
cost. And we could cut this constant by 4 orders of magnitude if only
I could safely differentiate between a put of a new entry vs. an
update -> something which we'll need to brainstorm about.

Query time is also significantly sub-linear in practice, but specifics
will vary on the query type.

Yes you could use indexes to improve x-cache joins, but you'll need an
additional engine to coordinate that correctly, not least to manage
data size buffers; essentially I think you'd need Teiid.