> Ten year old CPUs were plenty fast already,That depends on the CPU… Some are optimised for power consumption not performance, and on top of that will end up thermally throttled as they are often in small boxes with only passive cooling.
A cheap or intentionally low-power Arm SoC from back then is not going to perform nearly as well as a good or more performance oriented Arm SoC (or equivalent x86/a64 chip) from back then. They might not cope well with 2.5Gb networking unless the NICs support offloading, and if they are cheaping out on CPUs they might not have high-spec network controller chips either. And that is before considering that some are talking to the NAS via a VPN endpoint running on the NAS so there is the CPU load of that on top.
For sort-of-relevant anecdata: my home router ran on a Pi400 for a couple of years (the old device developed issues, the Pi400 was sat waiting for a task so got given a USB NIC and given that task), but got replaced when I upgraded to full-fibre connection because its CPU was a bottleneck at those speeds just for basic routing tasks (IIRC the limit was somewhere around 250Mbit/s). Some of the bottleneck I experienced would be the CPU load of servicing the USB NIC, not just the routing, of course.
> far more than enough even, to power an NAS device.
People are using these for much more than just network attached storage, and they are sold as being capable of the extra so it isn't like people are being entirely unreasonable in their expectations. PiHole, VPN servers, full media servers (doing much more work than just serving the stored data), etc.
> There must be more than that, another explanation
Most likely this too. Small memory. Slow memory. Old SoC (or individual controllers) with slow interconnect between processing cores and IO controllers. There could be a collection of bottlenecks to run into as soon as you try to do more than just serve plain files at ~1Gbit speeds.