The fastest internal SSD in a Mac now can write at 3.3GB/sec, so it would be able to keep up with 25Gbps connections, barely.

Average SSD write speeds are usually around 1GB/sec these days, but most folks could build a RAID of several SSDs and be able to keep up with such a connection, as well.

I would also assume that most folks who would bother to install 25Gbps, or anything close, probably have a lot of devices in their homes to take advantage of it at once.

Personally, I find it very hard to saturate the 1Gbps link I have. Most servers won't push more than around 200Mbps out to you, and I'm basically never doing 5 tasks at once that are that bandwidth-intensive. But it's nice to have some overhead, just for fun.

Modern SSDs can handle it: 25Gbps is ~3GB/s, which you can do with a PCIe 3.0 NVMe SSD. PCI 4.0 NVMe SSDs can do 5.1GB/s, or ~41Gbps.

I’ve had great results sticking a single Optane NVME drive in front of my 8x8GB array of spinning rust via ZFS’s built-in cache facility. I’m only doing write caching and only using 32GiB of my much larger drive (I forget exactly how large lol). That was my comfort zone for how much data I’m willing to lose if something very bad happens to the NAS during a write, and I like giving it a bunch of spare flash space for wear-leveling.

https://www.servethehome.com/exploring-best-zfs-zil-slog-ssd... is several years old but still a good read.

Hard drives don't go anywhere close, but SSD do. What is important to understand is that 25 Gbps Internet does not mean you copy from a server on Internet to your local SSD at that speed, that 25 Gbps is just the connection between your router and the ISP. On your side you may have multiple computers, on the ISP there are multiple peerings with other networks, you may reach and aggregate bandwidth of 25 Gbps but not really a point to point one, so your SSD performance is not the top factor.

I believe the transfer will simply be bottlenecked, like running a raspberry pi from an sd card.

To fully utilize the connection you can use storage backed by memory. For example ESXi allows you to back a vm by a "virtual pmem" disk that uses a chunk of your host system dram

You'd need raid SSD's or a sizeable RAM disk to make sustained use of it. Very few SSD's support full 25GB/s

I would imagine an NVMe SSD would be capable of handling most, if not all, of that throughput for sequential reads and writes.