XS512EM ports download speed

I am experiencing the same problem as you are, with similar hardware. Have you resolved it?

I am running 
Frontier provided FRX523  ONT (2Gb/s plan)
OPNsense firewall (Xeon E3-1270/16GB ram, 6x10Gbe)
Netgear XS512EM
My PC (2.5Gbe)

I get 2Gb/s down/up all day long connecting directly to the ONT. My firewall only negotiates 1/10gb, so my 2.5 will not provide more than 1Gb directly attached to it, but through the 10-Gigabit/Multi-Gigabit Plus (XS512EM)  my nic correctly negotiates 2.5Gb yet I max out at 1Gb/s down, 2Gb/s up going out to the internet

The strange thing is I hit full bandwidth if I hit my firewall using iperf, and/or if I connect to another server internal running OpenSpeedTest either on the same switch, or passing through the Firewall. It's only INTERNET traffic that suffers from reduced download bandwidth, and ONLY when passing through the XS512EM.

Troubleshooting steps I have done include the above connecting directly, swapping cables, ports, local iperf/speed tests, Fresh install of OPNsense and pfSense. Reflashed firmware (both 1.0.2.0 and 1.0.2.5) on the SX512EM, Factory Defaults... I don't know what I haven't tried at this point. The weird thing is it is only limiting my download to the internet. Almost like there's a TTL issue? At this point I don't know what I can do, but it's clear this switch is the common factor.