You'd be surprised. A single spinning rust drive can hit 200MBps for sequential access, so that's plenty to saturate its 1Gbps NIC.
However, in my experience with a Pi 4, the issue is encryption. The CPU simply isn't fast enough for 1Gbps of AES! Want to use HTTPS or SSH? You're capped at ~50Mbps by default, and can get it up to a few hundred Mbps by forcing the use of chacha20-poly1305. Want to add full-disk encryption to that? Forget it.
The Pi 5 is supposed to have hardware AES acceleration so it should be better, but I'm still finding forum posts of people seeing absolutely horrible performance. Probably fine to store the occasional holiday photo, but falls apart when you intend to regularly copy tens of gigabytes to/from it at once.
It apparently hit 387MBps for a few hours while running the montly raid scrub. I run luks on top of mdraid though so the raid scrub doesn't have to decrypt anything.
scp to write to the encrypted disk seems to get me something in the 60 - 100MB/s range.
However, in my experience with a Pi 4, the issue is encryption. The CPU simply isn't fast enough for 1Gbps of AES! Want to use HTTPS or SSH? You're capped at ~50Mbps by default, and can get it up to a few hundred Mbps by forcing the use of chacha20-poly1305. Want to add full-disk encryption to that? Forget it.
The Pi 5 is supposed to have hardware AES acceleration so it should be better, but I'm still finding forum posts of people seeing absolutely horrible performance. Probably fine to store the occasional holiday photo, but falls apart when you intend to regularly copy tens of gigabytes to/from it at once.