Shard per core has challenges. Scylla solves it partially by making the client shard aware (topology aware), so the client submits requests directly to the right cpu core using a different port. This way no data gets blocked due to hot keys or imbalance in other shards. You do need to have a good key distribution. We're implementing a new mechanism to split key ranges dynamically. It will be a fast an parallel mechanism.
If the number of connections is not small, Scylla will crank up any other implementation with traditional locking.
If the number of connections is not small, Scylla will crank up any other implementation with traditional locking.