mysql cluster insert is slower than mysql replication (no replies)

I am testing the MySQL cluster, set up two data node cluster, run a php script to insert 8000 records to one table, it take 10 seconds, while with mysql replication it only take 4 seconds, why cluster is so slow?

Cluster setting
[ndbd default]
NoOfReplicas=2
DataMemory=32G
IndexMemory=16G
DataDir=/var/lib/mysql-cluster