So I've grabbed the data from when the firewalls started to struggle again last week. It seems to be happening when people are actually working on the network, as we are using it as a test network at the moment and saw the performance get worse after we had people working there.
I've attached the files as I took screenshots this time.
Again, rebooting the firewall seemed to solve the issue. Any ideas? Does something 'build-up' on the firewalls which then clears after a reboot?