I timed sending 1450 bytes via socket send (it's a TCP Stream), on my K64, it can take up to 800 uS. I tried with OPT_SEND_NOWAIT, not using the push flag, and even directly setting the MSG_DONTWAIT in the send command and still see 800uS.
The timing was done on a scope (setting pin high/low before/after command).
I noticed that sock_stream.c, SOCK_STREAM_send calls RTCSCMD_issue which calls RTCS_msg_send_blocked in both the WAIT and NOWAIT state. There is an RTCS_msg_send command that isn't called. Not sure if this is why it's taking so long to return or is this normal?