It appears that on at least some machines, Snow Leopard set the default for Empty Trash to Securely Empty Trash, which involves a multi-pass erase scheme.
In the process of doing some performance tests on the DroboPro, I downloaded six files, each about 72 GB. When I tried to delete them, it took all day to delete just one! I have no idea how many erase passes they use – maybe the discredited 35-pass scheme that was recommended at one time.
To make matters worse, the Stop Erase command doesn’t seem to work, either.
However, it turns out that the default behavior can be specified in the Finder Preferences Advanced tab, where you can turn off Empty Trash Securely. You can still Empty Trash Securely if you need to, by going to the Finder menu.
Why this option was checked on my Mac Pro and not on my Mac Mini, I’m not at all sure. Maybe it only applies to certain device classes.