IRQ #105 seems to be the Ethernet controller interrupt.
Try this command with super user. I hope it will move the IRQ handler to CPU#2 from CPU#0.
echo 2 > /proc/irq/105/smp_affinity_list
So, the MTU size matters a lot, but the best value is odd. After all my tests I settled on an MTU size of 6975 (I was using 9000). It was the fastest and most stable of over 30 MTU sizes I tested. Why this value is the fastest I have no idea, but values between 6950-7050 are MUCH faster. For example, 7075 is over 20% slower than 7050. The performance just falls off a cliff once you go past a certain value and you can’t recover it. Likewise, values too low (under 6050) suffer lower performance in iperf, with speed decreasing as the MTU size gets smaller.
ifconfig eth0 mtu 6975 up