However, using the next iPhone or any other 5G-enabled device on today's network will be "like having a Ferrari...but using it in your local village and you can't drive up to 200 miles an hour, just because the roads can't maintain those speeds," said Boris Metodiev, associate director at research firm Strategy Analytics. Apple, which is expected to unveil the new phone at a virtual event on Tuesday, is walking a tightrope between consumers drawn to upgrading their phones and the risk of over-promising on 5G capabilities.

Current 5G networks in the United States primarily use low-band, or airspace, wireless spectrum, which is slower than high-band spectrum, but more reliable over longer distances. It will likely be years before the massive speed boost promised by phone carriers makes cloud-hosted augmented reality and real-time gaming accessible. Several US telecom operators have deployed networks based on lower frequency bands, with speeds slightly faster than 4G. A significantly faster variant of "mid-band" 5G is also rolling out, but it's unlikely to reach three-quarters of Americans before 2025, estimates longtime Apple analyst Gene Munster for venture capital firm Loup Ventures.

The fastest speeds touted by the carriers are a type of 5G called Millimeter Wave, or mmWave, which works over shorter distances. Verizon Communications has the largest mmWave network today, but only available in limited areas. Although Verizon 5G users will be able to connect nearly 10 times faster than on Sprint and T-Mobile US, actual average speeds are much lower, according to research firm OpenSignal's Mobile Signal Experience Report, published in June. On average, AT&T Inc and Verizon customers with 5G phones saw only a small increase in speeds over 4G, according to the same study.

In South Korea and China, faster 5G networks are more prevalent. But Apple will compete with local brands, including South Korea's Samsung, which is already on its second round of 5G phones, and Chinese company Huawei Technologies, whose sales surged after the telecoms giant was banned from United States.

The other big disappointment is that apps using higher speeds to deliver something new haven't been created yet. It's a chicken-and-egg problem at this point in the technology cycle. “Apps that blow us away, that really take advantage of the network, won't be developed until the network and the devices are available,” said Morgan Kurk, chief technology officer of CommScope, a telecommunications equipment maker.

The most motivated US consumers to buy a 5G phone would be those who need a new smartphone now and are looking to protect their investments. "If you're buying a phone that you're going to have for three years, you're going to want to make sure it's going to support the latest networks," said Geoff Blaber, vice president of research at CCS Insight. You just have to know what to expect, Blaber added. "There will be relatively few things you can do on a 5G iPhone that you can't do on a 4G iPhone today."