Yeah, Intel managed to incorrectly implement the 2.5GbE standard with the result that if you connected older steppings of their NIC to a standards-compliant device massive packet loss and effective data transfer speeds apparently as low as kilobytes/sec in some cases ensued. The only workaround was to force it to gigabit mode. They took an age to release a stepping that fixed this and I think there may have been other issues too, so a whole bunch of motherboards ended up shipping with 2.5Gb support that was effectively unusable.

To be honest, multiple vendors have really screwed up 2.5/5/10G implementations—I tried cheaping out with FlyProFiber copper transceivers, and would get a solid 2.3 Gbps one way, with 400-800 Mbps the other way. Turns out there was tons of packet loss, but only in one direction.

They worked fine for 10 Gbps, but I had to switch to Mikrotik transceivers for 2.5G connections.

Not all Realtek NICs are fine at this point in time. For some models of Realtek NIcs the Linux driver has to memcpy() data for every single packet in order to work around a hardware bug. It's entirely possible that the Linux driver has that kind of workaround enabled for newer hardware that doesn't need it anymore, but getting that kind of driver change merged tends to be a bit of an uphill battle without having the hardware manufacturer actively involved in maintenance of the mainline open source driver.

Interesting. I'm having a problem right now with a motherboard I just bought that has an Intel 2.5gb nic. Running Truenas connected to my gigabit switch I get a solid 110MB/s transfer but connected to my multi gig switch its down to around 45MB/s. It's a brand new board though.

Intel’s 2.5G is just an afterthought. Realtek pushes it because it’s more cost-effective than 10G but that’s where the actual party is at and where you’ll find good hardware and software support to match.. but at a price.