A problem exists in the LogiCORE Gigabit Ethernet MAC v5.0 Core when trying to transmit jumbo frames when the "Transmitter Jumbo Frame Enable" option is disabled. Specifically, the frame is truncated and GMII_TX_ER is asserted as expected, but the latency between tx_data_valid and tx_ack for the next frame becomes excessive (sometimes hundreds of cycles). This unnecessary delay consequently reduces data throughput rates.
This will be fixed in the Gigabit Ethernet MAC v6.0 Core which will be released in 7.1i IP Update #1.
To work around the issue until then, ensure that the Jumbo Frame option is enabled in the transmitter if jumbo frames will be involved.