Sharding is the process of splitting a database across many files. ZODB does not do sharding. If it were to do so, it would have to do two pahse commits across multiple files, and that woudl take even longer.
Jim Fulton writes:
"In particular, if transactions have to touch multiple shards, then shading will be slower. The only way sharding is going to be of much benefit is if transactions are unlikely to have to write to multiple shards and arranging that is hard."