system: 2x NICs with 10 Gb/s SFP+ ports connected with fibre-cable (Intel 82599, Mellanox MNPA19-XTR)
test-software: iperf3-3.1.3
result: +5%
#with mtu=1500 [SUM] 0.00-10.00 sec 11.0 GBytes 9.45 Gbits/sec sender [SUM] 0.00-10.00 sec 11.0 GBytes 9.40 Gbits/sec receiver #with mtu=9000 [SUM] 0.00-10.00 sec 11.5 GBytes 9.92 Gbits/sec sender [SUM] 0.00-10.00 sec 11.5 GBytes 9.90 Gbits/sec receiver
#e.g. change the mtu-value to 9000 ifconfig <your-device> mtu 9000