I remember in the Intel days, the Apple Thunderbolt 1 GbE adapter would have high CPU usage when you were transferring at the full 1 Gbps.

I've had good luck with the Realtek 2.5 GbE adapters, no CPU usage issues.

And these days even with a 10 GbE Thunderbolt adapter the CPU use is negligible, so things have improved across the board I think.