Check iperf performance

[peterharperuk]

Doing some testing in the office I find that tweaking the configuration gives a large improvement.

#define TCP_MSS 1460
#define TCP_SND_BUF (2 * TCP_MSS)
#define TCP_WND (4 * TCP_MSS)
#define TCP_SND_QUEUELEN ((4 * (TCP_SND_BUF) + (TCP_MSS - 1))/(TCP_MSS))

Also, disable IPV6 (junk broadcasts on the network?)

Freertos is giving me terrible performance compared to no sys. Is that reproducible with Pico W?