is send command in irules asynchronous

Question

I was trying to find out it in send command i can use async to improve the performance of the code

    when HTTP_REQ {
    set reqData [call req $myid]
    set send_bytes [send -status mystats $myconn $myresp]
    close $myconn 
}

tried placing async keyword inside

set send_bytes [send -async -status mystats $myconn $myresp]

However this ended in error.Any idea as to if the send command itself is asynchronous by default?