Trouble calculating web page load time

I agree with Hamish. I use HTTPWatch all the time, but if you want something to compare it to from the BIG-IP Perspective this iRule will track the TCP Connection (Start / Finish and provide Session Timing), as well as each HTTP Request and Response (with Request Timing).

 
when CLIENT_ACCEPTED {
Get time for start of TCP Connection in milliseconds
set tcp_start_time [clock clicks -milliseconds]

Log the start of a new HTTP Request
set local0. "(TCP Connection - Start) [IP::client_addr]:[TCP::client_port] to [IP::local_addr]:[TCP::local_port]"
}
when HTTP_REQUEST {
Get time for start of HTTP Request in milliseconds
set http_request_time [clock clicks -milliseconds]

Log the start of new HTTP Request with URI
set LogString "Client [IP::client_addr]:[TCP::client_port] -> [HTTP::uri]"

Log the start of new HTTP Request without URI
set LogString "Client [IP::client_addr]:[TCP::client_port]"

log local0. "(HTTP - Request) $LogString"
}
when HTTP_RESPONSE {
Recieved the response headers from the server.  Log the pool name, IP and port, status and time delta
Detailed Log Output - Client Communication - Pool Information - Status Messages - and Communication Delta
log local0. "(Response) $LogString - pool info: [LB::server] - status: [HTTP::status] (Request / Response Delta:  [expr {[clock clicks -milliseconds] - $http_request_time}]ms)"

Response and Communication Delta
log local0. "(HTTP - Response) $LogString - (Request / Response Delta:  [expr {[clock clicks -milliseconds] - $http_request_time}]ms)"
}
when CLIENT_CLOSED {
Log the end time of the TCP Connection
Detailed Connection Closed - Transaction can be located by using the Client IP Address and Port to Load Balancer IP Address and Port for additional matching parameters.
log local0. "(TCP Connection - Close) Client [IP::client_addr]:[TCP::client_port] to [IP::local_addr]:[TCP::local_port] (Connection Open for:  [expr {[clock clicks -milliseconds] - $tcp_start_time}]ms)"
}

Hope this helps!