This issue is described in detail in #64 under the heading TCP speed kludge, which essentially observes that introducing a delay in the ee16 driver's write select function will drastically increase send-speed.

What happens is that the delay prevents an extra round through select_wait/wakeup. A small delay causing a big speedup by 'disturbing' the transactional rhythm of the send process between ktcp and the driver. A complicated issue indeed because of the design of the system, but still one of great interest because of the improvement potential.

The 3C509 driver was just updated (by accident directly to master) with a delay similar to the ee16 driver and the ftp-send speed improvement was a factor of 4, sometimes beating 100kbps. This is remarkable - not the least because the max outgoing tcp packet size is 512, meaning there are a lot of packets processed.