While we haven't tested this, I suspect the main scaling bottleneck in practice is the primary database, as every Epoxy transaction goes through it. If you wanted to run Epoxy at large scale, you'd need a primary that supports very high transaction throughput.
More seriously, I suspect that plain SQLite by itself, running in process in the main service, writing to a ':memory:' database for ultimate speed or perhaps to a local file with WAL mode for slightly more durability, could achieve pretty nice efficiency and work as the main transaction manager database.
