I've got an aged Toshiba 1 GB drive in my A3k - close to 3 MB/s. The driver can only do async transfer, so ~4.5 is (very) theoretical max.
I've seen A590/A2091 do ~1.2 MB/s (with 50-200 MB drives 15 years ago), which isn't bad for a 68000 machine, but a faster CPU will make that much slower (due to bus sync overhead and/or driver issues).
