Here is an example of a USB to SCSI adapter. It has an advertised max transfer
rate of 1.2MB/sec, which is almost the USB 1.1 interface rate (i.e. not taking advantage of USB 2.0). If you were using a scanner, the transfer rate is low enough, to perhaps cause the scanner to enter start/stop mode (scanner stops while data buffer empties, increasing scan time).

http://www.coolgear.com/images/326605pro.jpg

The DB-25 option is something that Apple used to use. Typically this
is Async SCSI and 8 bit, running at a pretty low speed (5M/sec to 6M/sec,
with max speed proportional to cable length, due to how data strobing
works). Old scanning devices are seldom cable limited, and a scanner
with Mac DB-25 interface runs between 1 and 2MB/sec. The Coolgear example,
looks to have a DH50 connector.

http://www.datapro.net/techinfo/scsi_doc.html

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