mysqlcluster data nodes synchronization rate

[urgen sherpa]

mysql cluster version: MySQL-Cluster-gpl-7.4.12-1.el6.x86_64
num of datanode: 2
num of sql: 2
num of mgmt: 2

Recently one on my datanode server was down(node4) due to power outage. when i brought the disconnected datanode up i realized that the synchronization is very slow when it reached 17%.

[root@mgm1 ~]# ndb_mgm -e "all report memory"
Connected to Management Server at: localhost:1186
Node 3: Data usage is 18%(493874 32K pages of total 2621440)
Node 3: Index usage is 6%(71005 8K pages of total 1048608)
Node 4: Data usage is 17%(458693 32K pages of total 2621440)
Node 4: Index usage is 6%(70800 8K pages of total 1048608)

i checked the latest row on specific table and saw that the result is same which means result is same when accessing through both SQL nodes when table is updated.

Would you kindly enlighten me

Q1: How to know what is causing this slowness. I guess it is due to huge number of table indexes but not sure that index build up is taking this much time.

Q2: will it have negative impact if i do DML(insert/update) on SQL nodes when sync is on progress ?
note: (freeradius is connected to the sql nodes which constantly inserts data)