One of the main advantages of NEO over ZEO is its locking level.
The ZEO rule is simple, the whole database is locked at commit to prevent concurrent transactions to modify the same object. This is a bottleneck when dealing with many concurrent requests as all commits are serialized.
NEO locking system is fine-grained, a lock is held at the object-level when a store is requested. Thanks to that approach, two transactions that doesn't modify the same objects can be committed in parallel. This lead to a reduced commit latency and increased bandwidth.