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I've arrived into the US today by bus with a pre-filled I-94 so I can answer this.
You don't really save much time as the officer still asks you about your visit and you still have to submit your fingerprints. You maybe save a minute as the officer doesn't have to type in the address where you're staying in the US and another minute on not having to pay the $6 entry fee since you've already prepaid.
However I can still recommend applying in advance as it's one less thing to worry about at the border. Likewise you can pre-print the customs declaration to avoid filling it out with a pen.