@0n0w1c : According to Jeff Geerling’s notes on his (much more thorough and informed) 2.5GbE test, Jumbo Frames (MTU 9000) are the key to getting peak performance out of a 2.5GbE interface on the Pi.
Overclocking will provide some benefit, but MTUs should provide peak benefit.
So, I just tried it. I reset the MTU to 9000 on my Mac (the iperf3 server), the Pi’s USB 3.0 ethernet adapter, and my NAS (which the Pi spends a lot of time talking to because deja-dup and timeshift together mean my Pi spends most of its time backing something up). The switch is a multi-gig model, that is set to MTU 9000 by default and apparently does something to auto negotiate with smaller frames. Unclear on this, but tl;dr it defaults to MTU 9000.
Result: Performance is unchanged, but iperf3 no longer pegs CPU core 1. Instead, it gets around 40-60 percent on core 1, and 30-50 percent on core 2.
As I’m not a fan of pegging CPUs, I think this is worth changing the MTU settings for, even if it’s not needed for speed.