We have a Balance380 with 5.4.7 build 2423 firmware. We do not use any kind of group policy QoS. We have two WAN links:
Fios at 150/50
Cable at 50/10
Today, a user was downloading software updates from our Fios connection at about 75Mbs, and this caused CPU load to spike to 100% on the router. Admin page requests started to fail and internet generally became unavailable to other users on our network. The the 380 should be able to sustain this throughput without falling over.
Please note that the Balance 380 has a router throughput rating of 200 Mbps. The sum of your WAN upload and download speeds has the potential to exceed that and may cause undesirable behavior. You are correct the Balance should sustain 75 Mbps of throughput. In order to investigate what is happening I would recommend opening a support ticket with us here.