Earlier quoted context omitted.
It's not throttling based on user-agent. It's just a bug that happens when the user-agent parser falls off a cliff when it exhausts the values that it knows about. A bug that Microsoft won't fix, because Linux is not supported.
But user-agent parsing should be blazing fast, a one-liner regexp or similar, isn't it?
case @useragent
when 'Windows'
send file fast
when 'OSX'
send file fast
else
sleep 3600