This is not my experience.

I have used many 1000BASE-T dongles and they work exactly as advertised - capable of transferring at ~950Mbps.

I have also used 2.5GBASE-T dongles and speeds are in the 2Gbps+ range.

WisdPi are even offering dongles with 5GBASE-T support (RTL8157 chipset):

https://www.wisdpi.com/products/wisdpi-usb-3-2-5g-ethernet-a...