Communication apparatus includes a memory, which is configured to hold data packets, having respective packet sizes, for transmission over a data link, and a transmitter, which is configured to transmit the data packets over the data link at a bit rate determined by a wire speed of the data link. A shaper is coupled to throttle transmission of the data packets by the transmitter responsively to the respective packet sizes, whereby some of the data packets are transmitted over the data link at a transmission rate that is less than the bit rate.