When Apple launched the iPhone 4S without 4G, competitors such as Samsung mocked its lack of a modern, whiz-bang broadband network. It was no surprise, then, that Wednesday’s launch of the new iPad focused on what was underneath the hood of the top-of-the-line version: a high-octane 4G LTE data connection. Translated from mobile-ese, that means really fast Internet, available on the go—not just in Wi-Fi hot spots. “The performance is amazing, and you’re going to love using it on these new high-speed networks,” company marketing honcho Philip Schiller promised.

Before you throw your iPad 2 out the window, you should know one thing about the mobile information superhighway: Life in the fast lane ain’t cheap. With an advertised top download speed of up to 73 megabits per second, the top-of-the-line iPad could theoretically burn through a basic 2-gigabyte-per-month data plan in about four minutes. But oh, how delicious those four minutes would be!

At Wednesday’s launch, Schiller took the new 4G iPad for a quick spin through some streaming video, showing how much faster it loaded than on a 3G network (about 10 times faster, he said). His tech-blogger audience was duly impressed, though the irony of this moment was not lost on everyone. (“It’s like a Samsung commercial on the Apple stage,” tweeted the Huffington Post’s Jason Gilbert.) Speed isn’t the device’s only virtue. The new iPad also has a ludicrously high-resolution screen, which Cupertino’s marketers have dubbed a “retina display.” Add it all up, and you have a high-performance online gaming or video-watching machine.

Just be careful how much you indulge in that immersive experience, or you might find your first AT&T or Verizon bill unexpectedly immersive as well. The data plans for the third edition of the iPad will have the same costs and data limits as the carriers’ current plans. For Verizon, that’s $30 for two gigabytes per month, $50 for five gigs, and $80 for 10 gigs; AT&T’s prices are similar. Neither offers an unlimited plan anymore, with AT&T recently following Verizon in canceling its last all-you-can-stream options. (Sprint, the last carrier to offer unlimited data, was left out of the latest iPad launch, notes industry blog FierceWireless.)

That’s pricey enough that many users of the next-gen iPad will likely ignore its mobile data capabilities. Though the 4G LTE network is one of the new gadget’s big selling points, the reality is that the majority of tablet users connect to the Internet where Wi-Fi is available, download what they need, and spend the rest of their time offline. Given how data plans work, that approach makes a lot of sense. The 4G device itself also costs $130 more than the basic iPad, with a starting price of $629.

It is admittedly unlikely that anyone would max out their data plan within four minutes. For starters, peak download speeds are theoretical. You’d probably have to be standing on top of the cell tower, and be the only person using it, to reach 73 Mbps. Moreover, such speeds are unnecessary for most purposes. High-definition streaming video, for instance, requires about 50 Mbps, according to industry strategist Chetan Sharma. But your average YouTube video runs at more like 1 Mbps, and run-of-the-mill Web browsing uses much less than that. In those cases, a five-gig plan would let you watch or browse for many hours before you hit your limit. Start downloading movies in your car, though, and you’ll probably hit your limit during your second full-length feature.

And once you hit that cap, it would be wise to hit the brakes and pull over. Based on AT&T’s overage charge of $10 per gigabyte, you could plausibly incur $100 in charges in a single hour. Of course, your carrier would shut you off long before that happened. Thank goodness for throttling.