A few suggetions:

1- make sure replication is on a dedicated link, not the same IPs as iSCSI traffic.
2- tune the targets for performance.

These settings work well for most VM hosts:
1. From the console, press CTRL+ALT+W,
2. Select Tuning options ->
iSCSI daemon options -> Target options,
3. Select the target in
question,
4. change the MaxRecvDataSegmentLength and MaxXmitDataSegmentLength
values to the
maximal required data size (check w/ the initiator to
match).

maxRecvDataSegmentLen=262144
MaxBurstLength=16776192
Maxxmitdatasegment=262144
FirstBurstLength=65536
DataDigest=None
maxoutstandingr2t=8
InitialR2T=No
ImmediateData=Yes
headerDigest=None
Wthreads=8
3- expand the timewait settings in the failover configuration, and make sure ping times to the ping nodes are fast.
4- Use jumbo frames