After installing the IBM PRO/1000 PT Quad NIC, I was able to achieve full speeds.
Various issues were present when I swapped the NICs. The labeling of the interfaces in Proxmox was different, and there was no way of distinguishing between the 4 ports without testing each port with static bridge IPs.
The motherboard NIC wasn’t working afterwards for some reason, but 4 ports takes care of that issue.
The internet speeds reverted back to what I expected: >150mbps download and full 15mbps upload. The speed was achieved without the use of jumbo frames (10k). As a result, packet loss is virtually gone. DHCP managed to get the MTU size of a standard 1500 bytes.
The latency is as expected but the biggest improvement is in QoS. Before I deployed the UTM VM as the gateway/firewall, games constantly experienced large ping spikes ranging from 200ms to 600ms – literally unplayable. I was able to set up rules for game ports to get guaranteed bandwidth, and also reducing max upload speeds based on application from single sources. This reduced the ping spikes to about 20-40ms.
Many features were great to use. I was able to implement IPS protection and already prevented two attacks. Monitoring bandwidth usage and interface utilization were the most used since deploying this appliance.
A feature I want to take advantage of now is to filter ads through DNS blocking, but I need to find a way to use public lists to filter against.