public static void testWithoutCallout(final Id leadId)
{
    System.debug(DateTime.now().format('m:s') + ': Lock requested');
    final Lead lead = [Select id From Lead where id = :leadId FOR UPDATe];
    System.debug(DateTime.now().format('m:s') + ': Lock acquired');
    for(Integer i = 0; i < 30000000; i++) {}
    System.debug(DateTime.now().format('m:s') + ': Done');
}

public static void testWithCallout(final Id leadId)
{
    System.debug(DateTime.now().format('m:s') + ': Lock requested');
    final Lead lead = [Select id From Lead where id = :leadId FOR UPDATe];
    System.debug(DateTime.now().format('m:s') + ': Lock acquired');

    final HttpRequest req = new HttpRequest();
    req.setEndPoint('https://www.google.de');
    req.setMethod('GET');
    final Http http = new Http();
    http.send(req);

    for(Integer i = 0; i < 30000000; i++) {}

    System.debug(DateTime.now().format('m:s') + ': Done');

}

Execution 1:
16:56: Lock requested
16:56: Lock acquired
17:02: Done

Execution 2:
16:59: Lock requested
17:59: Lock acquired
17:17: Done

Execution 1:
21:18: Lock requested
21:18: Lock acquired
21:34: Done


Execution 2:
21:23: Lock requested
21:23: Lock acquired
21:38: Done