Another data point, the new
Belkin USB Type C to 2.5 Gb Ethernet Adapter works. When attached to Studio Display, this can reach 1 Gb speed (don't have 2.5 to test). Notice, this adapter is larger than the 1 Gb version
I suspect it's a driver issue, because this one uses a different driver than the Belkin 1 Gb version from Apple Store (see below).
This is from System profiles
Vendor Name: Realtek
Product Name: USB 10/100/1G/2.5G LAN
Vendor ID: 0x0bda
Product ID: 0x8156
USB Link Speed: Up to 5 Gb/s
Driver: com.apple.driver.usb.cdc.ncm
Maximum Link Speed: 2.5 Gb/s
This is the same from the 1Gb version
Vendor Name: Belkin
Product Name: Belkin USB-C LAN
Vendor ID: 0x0bda
Product ID: 0x8153
USB Link Speed: Up to 5 Gb/s
Driver: com.apple.DriverKit.AppleUserECM
There is
more discussion here "USB CDC-NCM, packets take a much more direct route (through kernel space once only), which is hardware-optimised – meaning the route is faster (enabling lower latency settings), and inherently more stable. "
Hope this helps