Does iPhone 12 Pro support gigabit Ethernet speeds?

From my research, the answer is no. This is a hardware limitation of the Lightning port, which is limited to USB 2.0 speeds (480 Mbps). And I believe Lightning to Ethernet speeds are limited to 100Mbps.

The iPhone doesn't technically "support" ethernet at all. it does not have an Ethernet port. Apple doesn't claim it can use an Ethernet connection.

Why anyone would want to do this in the first place on a phone is beyond me.

There are Mfi-certified Lightning to Ethernet adapters available. As for how fast an adapter is, try it. Given few (two?) Lightning configurations were faster than USB 2.0 (and iPhone 12 Pro wasn’t one of these), I wouldn’t be hopeful.

Wi-Fi 6 networks can be faster than gigabit. In the case of a 2x2 with 160 MHz slots, Wi-Fi 6 deliver roughly a gigabit. In more than a few of cases there, the bottleneck is the speed of the selected access point, or of its wired backhaul link.

If you’re finding wired Ethernet is faster than Wi-Fi, then configuration issues, interference or contention, or a Wi-Fi router hardware issue, are all possibilities, as well.

It’s far easier to test the Wi-Fi with a Mac or Windows PC or Linux, as the iPhone tools are very limited. Do you have a Mac available?

Probably not.

use case: i have success using ethernet when my broadband is too slow

The question isn’t supporting a connection. With a good adapter, thats likely. The question is supporting gigabit speeds.