Delphi: Why does IdHTTP.ConnectTimeout make requests slower?

Question

I discovered that when setting the ConnectTimeoout property for a TIdHTTP component, it makes the requests (GET and POST) become about 120ms slower?

Why is this, and can I avoid/bypass this somehow?

Env: D2010 with shipped Indy components, all updates installed for D2010. OS is WinXP (32bit) SP3 with most patches...

My timing routine is:

    Procedure DoGet;
    Var
       Freq,T1,T2 : Int64;
       Cli        : TIdHTTP;
       S          : String;
    begin
         QueryPerformanceFrequency(Freq);
         Try
            QueryPerformanceCounter(T1);
            Cli := TIdHTTP.Create( NIL );
            Cli.ConnectTimeout := 1000;  // without this we get < 15ms!!
            S := Cli.Get('http://127.0.0.1/empty_page.php');
         Finally
            FreeAndNil(Cli);
            QueryPerformanceCounter(T2);
         End;
         Memo1.Lines.Add('Time = '+FormatFloat('0.000',(T2-T1)/Freq) );
    End;

With the ConnectTimeout set in code I get avg. times of 130-140ms, without it's about 5-15ms ...

Answer 1

When ConnectTimeout is zero (and TIdAntifreeze is not in effect), Indy simply connects. Otherwise, TIdIOHandlerStack.ConnectClient calls DoConnectTimeout, which creates a new thread to do the connecting while the calling thread sleeps and processes TIdAntifreeze operations, waiting for the connection to be established. If there's not connection by the time the timeout elapses, it throws an exception.

Threads aren't free, and the calling thread will always sleep before checking whether the connection thread has accomplished its task. The default sleep duration is 125 ms. (To use something else, activate TIdAntifreeze and set its IdleTimeout property lower than 125.)