10Gbps internet connection isn't maxing out 2.5Gbps network card?

I have a 10Gbps internet connection. On a system with a 10Gbps Ethernet card, I can get ~8Gbps down and ~6Gbps up:

I'd expect this to easily max out a 2.5Gbps network connection. However, while the upload is maxed (or close to it), I can only ever get ~1.0 to 1.5Gbps down:

Both tests were performed on the same system. The only difference is that the first one uses a TRENDnet 10Gbps PCIe network card (which uses an Aquantia AQC107 chipset) whereas the second one uses the onboard NIC on my motherboard (Intel I225-V chipset).

This is consistent across two devices that have 10Gbps ports and two devices that have 2.5Gbps ports.

I'm using an AdTran 622v ONT provided by my internet provider, a TP-Link ER8411 router, and a MikroTik CRS312-4C+8XG-RM switch. I'm using CAT6 cabling, except for the connection between the router and the switch which uses an SFP+ DAC cable.

I haven't been able to figure it out. The 'slower' speeds are still great, I just don't understand why it can't achieve more than 1.5Gbps down over a 2.5Gbps network connection.

Any ideas?