Yup, the old Ethernet lib was horribly inefficient on every level.
Due's SPI library is still very inefficient, which holds back Due's performance to ~700 kbytes/sec. If someone were to improve the SPI lib, I believe Due could probably even outperform Teensy (where the SPI lib is highly optimized) on these tests, because Due is the only board that can actually produce a 14 MHz SPI clock. Pretty much all the others use 8 or 12 MHz when SPISettings asks for 14 MHz max.
The SPI lib on Due isn't my project. My dev cycles are funded by Teensy sales. All this optimization work came from Teensy's fork of Ethernet. Occasionally I try to contribute Teensy's improvements back to the rest of the Arduino community, so everyone can benefit. Hope everyone gets some good use from it. 