I'm looking at buying a scanner with a document feeder. The feeder can feed in 30 ppm, but the scanner can only can at 12 ms/line. So how fast is the scanner scanning? Is having such a fast document feeder useless?
Well an A4 page (I live in the UK) has about 50 lines of text on it, so to do an A4 page it would take 12 x 50msec = 600msec or a little more than half a second. A 30 ppm sheet feeder would feed one every two seconds leaving you with a reasonable margin for error.
I would suggest you only use the numbers as a guideline, see if you can find a review or some real world tests. Find out what people actually think about it, Amazon.com is quite good for user reviews of consumer products.