Response time iRule error

If your goal is to simply check the server's response time to a request, I'm including a small snippet of what I have done in the past.

Probably the best time to capture the actual response time is between HTTP_REQUEST_SEND and HTTP_RESPONSE.

ltm profile statistics DELAY_STAT {
    field1 total
    field2 webtest_total
    field3 webtest_slow
    field4 webtest_fast
}

ltm rule LOG-Delay {
    when RULE_INIT {
     what is a slow response in milliseconds
    set static::slow_response 2000
}
when HTTP_REQUEST_SEND {
    STATS::incr DELAY_STAT total
    clientside {
        if {[string tolower [HTTP::host]] contains "onpoint342labupgrade2013"} {
            STATS::incr DELAY_STAT webtest_total
            set URL [string tolower [HTTP::host]][HTTP::uri]
            set SENT [clock clicks -milliseconds]
        }
    }
}
when HTTP_RESPONE {
     exit event if variable is not set
    if {![info exists SENT]} { return }

    set TIME [expr {[clock clicks -milliseconds] - $SENT}]
    if {$TIME >= $static::slow_response} {
        STATS::incr DELAY_STAT webtest_slow
        log local0. "[HTTP::status]: Client [IP::client_addr]; \
        Server [IP::server_addr]:[TCP::server_port]; URL ${URL}; Time ${TIME}ms"
    } else {
        STATS::incr DELAY_STAT webtest_fast
    }
    unset SENT URL TIME
}
}

As long as HTTP pipelining is not being used, and hopefully it's not, this would be fine.

Notes

1. If you expect a lot of logging, I recommend using HSL instead of the log command.
2. I'm not logging every bit of information, you can add as needed.
3. You should check out the request logging profile on the F5. It may be able to accomplish what you are looking for without an iRule, though I don't think you can control which URLs it is logging.
4. I added STATS to track all/slow/fast requests. You would need to add the stats profile to the virtual server for it to work. In the past, I have found it handy to see how many requests were fast vs slow at a glance. Additional STATS fields can be added fairly easily to track other things without requiring you to sift through log messages.
5. The delay between HTTP_REQUEST_SEND and HTTP_RESPONSE may not include the total response time. The HTTP_RESPONSE event fires when the F5 has received the complete HTTP header, which is not always the same as the response contents.