Even if the initial congestion window is set to 10 segments, all that does is send out the packets if they're available to send. It still takes less time for the receiver to receive 3 segments than 10 segments, having the congestion window be that wide just means that the sender doesn't need to wait to receive ACKs for those 10 segments (which, to be fair, is faster than sending each packet and waiting for an ACK.)