Are the connections shown in your last screenshot of the actual port 9020 backup connections (and not iperf3)? If so they are in the fastpath already and will not cause the fast_accel hit counter to go up. The blade-based exception you created for IPS and port 9020 has probably made this traffic eligible for the fastpath already.
When the backup is running, do any of your gateway's SND cores hit 100%? If they don't something else other than the firewall is slowing down that backup traffic (possibly fragmentation), for the gateway please provide the output of netstat -ni for the relevant interfaces the backup traffic is traversing to look for network problems.
Just because the server being backed up has a 10Gbit NIC doesn't mean the backup software can actually push traffic at 10Gbit, you may want to look at resource utilization on the server being backed up and see if the backup software is running out of CPU at 700Mbit. Not impossible if it is encrypting the backup traffic but is limited to a single thread/CPU. Also look at your backup server while this backup is running and make sure it has plenty of resources and is not throttling the inbound backup traffic.
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