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What voltage are you feeding it from?, the L239 drops a LOT of voltage if I recall correctly?.
A normal stepper motor should do a revolution per second without taking any special measures.
Here's what I've used various times:
Using a 12V stepper motor (from a 5.25" floppy) running off the 5V supply, easily gives one rev per second.
Try decreasing the stop delay slowly, and make sure your step sequence is correct.
The L239D is more suited for bipolar steppers, but as far as I'm aware it can be used for unipolar as well, although you're wasting half of the chip. Post a drawing of how you've connected it all up.
I see you've now changed the number of the driver chip
The circuit looks reasonable, what about your code?.