@CalmDot -- do you have a CAT-5e or CAT-6 or CAT-8 Ethernet cable between your computer and the cable-modem? One of those cables maxes-out at 1.0 Gbps, as you have observed.
On your computer, check the "network properties" for your 2.5 Gbit Ethernet adapter, to ensure that it is sending/receiving at its maximum speed -- not at 1.0 Gbps.
Note that 10 IP packets (sent at the 1.5 rate with no delays between packets) to your cable-modem should each be sent at a 2.5 rate to your computer, but those 10 packets will take a shorter time to reach your computer, namely only 1.5/2.5 (60%) of the elapsed time that it took those incoming packets to reach your cable-modem.
Of course, you need to be accessing some server on the Internet that can send at 1.5 Gbps per second. If that server only has a 1.0 Gbit Ethernet adapter, that's the rate (not the speed) that the packets will reach your computer.
For me, somewhat physically close to Microsoft's [regional] "Windows Update" server near Seattle, my 1 Gbps connection never receives at over 500 Mbps from that site.
Rogers now offers 2.0 Gbps Internet. It is an option for people wanting two computers in their home to each use the Internet at 1.0 Gbps without interfering with each other. Rogers 1.5 Gbps service allows three computers to each get 500 Mbps at the same time.
