For some reason, the default
server.max-write-idle = 360
is too short... I've increased that to 720 and it's still too slow according to the error log.
I'll give:
server.max-keep-alive-requests = 4
server.max-keep-alive-idle = 4
a try then.

I'll report back.
In relation to kqueue, I'm using FreeBSD so I have this line:
server.event-handler = "freebsd-kqueue"
That covers that yeah?