T-Mobile's nationwide 5G network will start out 25-50 percent faster than its existing 4G LTE network, the carrier said today. That's much slower than the multi-gigabit speeds AT&T and Verizon have been touting, but those carriers largely plan to use very different spectrum.

T-Mobile is mostly going for range with its nationwide 600MHz because it can cover huge distances; AT&T and Verizon will generally do smaller hotspots using millimeter wave.

According to Karri Kuoppamaki, T-Mobile's VP of radio network technology development and strategy, higher-frequency millimeter wave spectrum only has cells about 900 feet wide, though Verizon's chief network officer Nicola Palmer said last year that they can get gigabit speeds at 2,000 feet from a cell.

"900 feet. If that's your cell radius, that's about 0.1 square miles, and the continental US is over 3 million square miles," Kuoppamaki said at the Brooklyn 5G Summit today.

T-Mobile will, however, supplement its 600MHz network with millimeter wave in high-speed hotspots.

But the value in low-band 5G isn't its initial state; it's the ramp-up. 5G technology has a lot more room to grow than 4G, even if it's starting out just a bit ahead of current speeds.

"Initially, we didn't see gigabit speeds on LTE; we saw very low speeds. But today we see much higher than that. It's kind of irrelevant what [the speed] number is going to be on day one, as it will improve over time," Kuoppamaki said.

That's a similar message to what his boss, T-Mobile CTO Neville Ray, said when he announced the carrier's 5G rollout at Mobile World Congress in late February.

"Are we going to see average speeds start to move up by tens of megabits per second? For sure," Ray said at the time. "We would love to see average speeds triple, or move to 100Mbps, but that's a journey that's going to take time in the industry."

LTE will also continue to develop, and carriers will keep using LTE where it's a better fit. "LTE and 5G, they're conjoined technology twins that will happily coexist in the foreseeable future," Kuoppamaki said.

T-Mobile is starting to build 5G in about 30 cities this year, including New York, Las Vegas, Los Angeles, and Dallas, using a "mix of low-band and high-band" spectrum, Kuoppamaki said. Ray said at MWC that we'll see T-Mobile 5G devices, including smartphones, in 2019.