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...

USB-to-Ethernet adapters are life savers when you need to:

(A) replace your WiFi adapter - download drivers from internet

(B) configure a router or other equipment (hard to configure WiFi without WiFi).

(C) stand up your Linux install on your laptop (easiest way to futz around until you get WiFi adapter working - but check chipset on adapter is compatible which the cheapest usually are)

You don't usually care about the performance. Just keep a cheap one in your box of shit - I need mine often enough. If you need high performance, then buy a high performance adapter.

Old custom software, old hardware, vendor wants all the $ for an upgrade, we refuse to pay. I took 10 desktop pc's($500 each) replaced servers ($20k each), one usb to ethernet dongle in every pc b/c we needed 2 network ports and we had this laying around, USB3 to GB, slap virtualization with USB passthrough. They work for 5+ years, gigabit speed, 24/7 with no problems.

People should have more faith in dongles. Not all are bad.

In my experience they always held up the 100 Mbit/sec claim for lower-end variants, and an acceptable 350-ish Mbit/sec on USB2-backed GbE devices. I have no experience with GbE USB3 dongles.

RTL8156B does line-rate 2.5 Gbit/s no problem, most USB-C docks with network have a RTL8153B in them and that does line rate as well. Even mildly dodgy first-generation stuff like AX88179 generally works.

I.M.H.O. these USB dongles are actually preferable to the much more expensive Thunderbolt dongles praised below, because a) they work on regular USB ports as well b) they do not require Thunderbolt c) they use less power and d) they don't force a highly ventilated cooling mode on certain host systems. And, fwiw, at least some Thunderbolt docks actually used USB NICs connected to the internal USB controller, which was hooked up over PCIe.