If you use a couple of magnetic disks, the pi is fast enough. The disks will be the bottleneck. There are sata cards that allow up to four magnetic disks, and where you power that card which in turn powers the pi. It's very doable.
It's of course more work to set up than synology, and if you want a neat box, you have to figure that out yourself
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.
So long as the storage system is capable of serving a video stream without stuttering, that covers the 99% performance case for me. Anything beyond that is bulk transfers which are not time sensitive.
It's of course more work to set up than synology, and if you want a neat box, you have to figure that out yourself