SATA 3 can move 500MB/s, but high-capacity drives typically can't. They are all below 300MB/s sustained even when shiny new. Look for example at the performance numbers quoted in these data sheets [1][2][3][4], all between 248 MiB/s and 272 MiB/s.
Now that's still a lot faster than 100MB/s. But I have a lot of recertified drives, and while some of them make the advertised numbers some of them have settled at 100MB/s. You could argue that is something wrong with them, but they are in a raid and I don't need them to be fast. That's what the SSD cache is for.
1: Page 3 https://www.seagate.com/content/dam/seagate/en/content-fragm...
2: Page 2 https://www.seagate.com/content/dam/seagate/en/content-fragm...
3: Page 2 https://www.westerndigital.com/content/dam/doc-library/en_us...
4: Page 7 https://www.seagate.com/content/dam/seagate/assets/products/...
Spinning rust drives tend to be much faster on the outer than the inner tracks.
I had a 12 disk striped raidz2 array comprised of wd gold drives that could push 10Gbit/s over the network, while scrubbing, while running 10 virtual machines, and still had plenty of IO to play with. /shrug