cluster network is for osd->osd replication.
osd->monitor && osd->client is done on public network.
They are differents hearbeats, osd hearbeat between osd (on clusternetwork)
hearbeat from monitor-> osd (on public network)...
your pg number seem quite low, it could impact write performance with pg_lock.
try to set pg target ratio to 1.0 in your ceph pool
Also, verify in your bios that your server is set to max performance profile. (you want cpu always at their max...