I have some 20 GB IBM SCSI drive in my Sun SparcStation 20 (from 1994, so even newer than your DEC 3000) and it only gets off about 3 MB/s (dd if=/dev/... of=/dev/zero). You can generally speed up dd by using appropriate bs=... parameters (512, ..., 4069 are good values as they are typical inode sizes). By using count=... you can benchmark before doing the full task.

According to math rules at 5 MB/minute you'll have to wait two days.