Why does transfer speed drop when I send and receive at the same time?

My understanding from the wikipedia page for Duplex and from StackExchange answers like this is that on a modern ethernet connection the **transmit** traffic is supposed to be unaffected by the **receive** traffic. But I'm running some tests between two modern Mac computers and I'm seeing results that make me think there's something crucial I'm missing in my understanding. The test rig is two Mac Studio M2 Ultra boxes, one with Ventura v13.7.4 and one with Sonoma 14.7.4 In order to reduce complexity, I've connected them directly to each other with a short CAT6 copper ethernet cable between their built-in NICs. When I let it auto-negotiate the speed it says 10GbaseT as expected, and for part of the test I set it to manual and 1000baseT System Preferences > Network > Ethernet > Hardware (and ifconfig) say Full Duplex in both modes - At 10GbaseT, using a finder copy on Mac A, copying from the local drive on mac A to the local drive on mac B, I get ~1200 MegaBytes/sec as expected (transmit-only) - At 10GbaseT, using a finder copy on Mac A, copying from the local drive on mac B to the local drive on mac A, I get ~1200 MegaBytes/sec as expected (receive-only) - At 10GbaseT, using a finder copy on Mac A, copying from the local drive on mac B to another share of the local drive on mac B, I get **~650 MegaBytes/sec** (simultaneous transmit/receive, **a little over half speed**) [I get similar results running two copies at once, one transmitting and one receiving] - At 1000baseT, using a finder copy on Mac A, copying from the local drive on mac A to the local drive on mac B, I get ~120 MegaBytes/sec as expected (transmit-only) - At 1000baseT, using a finder copy on Mac A, copying from the local drive on mac B to the local drive on mac A, I get ~120 MegaBytes/sec as expected (receive-only) - At 1000baseT, using a finder copy on Mac A, copying from the local drive on mac B to another share of the local drive on mac B, I get **~65 MegaBytes/sec** (simultaneous transmit/receive, again **a little over half speed**) [Again, similar results running two copies at once, one transmitting and one receiving] What gives? It's not just Finder, because terminal commands to copy do the same thing. It's not crappy third party hardware, I'm using the built-in ethernet. Even if there was some hardware limitation in the NIC, or a problem with storage speed, wouldn't it at least be able to get full 1GbE speed simultaneously sending/receiving in 1GbE mode? The reason we went testing is because we're seeing similar performance using switches and connecting to an actual fileserver, the test rig was just to eliminate variables. Auto-negotiating to a single 1GbE switch in between the two test boxes didn't fix it, and I tried NFS which didn't seem to make much difference. Jumbo Frames (MTU 9000) helped somewhat, it got up to about 85 MB/sec, but it didn't go all the way to the same speed as one-way copies, and we can't use Jumbo Frames in production unfortunately anyways.